Reduce or eliminate warnings on GCC and Clang

[pow2clk]

By my count, GCC produces 10126 warnings when building dxc. Clang produces 2335. (Clang has a tendency to consolidate when a long series of warnings are related) This overwhelming volume of warnings makes it difficult to recognize new warnings that might signal subtle bugs, slows the build, and brushes against the storage limits of the automated build system. The biggest !/$ were already eliminated as a precursor to enabling GCC builds, but there are still way too many.

This issue will serve to unite a few changes to fix various instances of specific warnings grouped into changes by type in hopes of making what would otherwise be a staggering monolithic change into comprehensible and reviewable pieces.

[antiagainst]

Agreed. I'm using Clang 6.0.0 locally and collected the following statistics:

For warning categories:

583 [-Wgnu-zero-variadic-macro-arguments]
170 [-Wgnu-anonymous-struct]
143 [-Wmissing-braces]
133 [-Wswitch]
121 [-Wunused-value]
101 [-Wcast-qual]
 82 [-Wunused-function]
 78 [-Wunused-private-field]
 78 [-Winconsistent-missing-override]
 69 [-Wmismatched-tags]
 69 [-Wmacro-redefined]
 47 [-Wunused-variable]
 34 [-Wcovered-switch-default]
 28 [-Wunused-const-variable]
 24 [-Wreorder]
 18 [-Wlogical-op-parentheses]
 11 [-Wsign-compare]
  8 [-Wdangling-else]
  6 [-Wunused-lambda-capture]
  5 [-Wundefined-internal]
  4 [-Wunneeded-internal-declaration]
  4 [-Wundefined-bool-conversion]
  4 [-Wmissing-field-initializers]
  3 [-Wpragma-once-outside-header]
  3 [-Wextra-tokens]
  3 [-Wenum-compare-switch]
  3 [-Wbraced-scalar-init]
  2 [-Wparentheses]
  2 [-Wformat-pedantic]
  2 [-Wdelete-incomplete]
  2 [-Wchar-subscripts]
  1 [-Wtautological-undefined-compare]
  1 [-Wsometimes-uninitialized]
  1 [-Wsentinel]
  1 [-Wformat-security]

For files triggering warnings:

721 ../include/dxc/Support/Global.h
255 ../include/llvm/Support/WinTypes.h
151 ../lib/HLSL/DxilOperations.cpp
 86 ../tools/clang/lib/Sema/SemaHLSL.cpp
 68 ../tools/clang/include/clang/Sema/Sema.h
 56 ../include/dxc/HLSL/DxilPipelineStateValidation.h
 54 ../lib/HLSL/DxilRootSignature.cpp
 36 ../include/dxc/Support/dxcapi.impl.h
 18 ../tools/clang/lib/SPIRV/SPIRVEmitter.cpp
 18 ../tools/clang/include/clang/Frontend/CompilerInstance.h
 18 ../include/llvm/Support/WinFunctions.h
 16 ../tools/clang/lib/CodeGen/CGHLSLMS.cpp
 15 ../lib/HLSL/DxilValidation.cpp
 14 ../lib/DxcSupport/FileIOHelper.cpp
 11 ../tools/clang/tools/dxcompiler/dxcdisassembler.cpp
 11 ../tools/clang/lib/SPIRV/TypeTranslator.cpp
 10 ../tools/clang/lib/AST/ASTContextHLSL.cpp
  8 ../tools/clang/lib/SPIRV/DeclResultIdMapper.cpp
  8 ../tools/clang/lib/Sema/SemaChecking.cpp
  8 ../lib/Support/APFloat.cpp
  8 ../lib/HLSL/HLSignatureLower.cpp
  7 ../tools/clang/lib/Sema/SemaDeclObjC.cpp
  7 ../tools/clang/lib/Frontend/ASTUnit.cpp
  7 ../lib/HLSL/HLMatrixLowerPass.cpp
  6 ../tools/clang/tools/dxcompiler/dxclibrary.cpp
  6 ../tools/clang/tools/dxc/dxc.cpp
  6 ../lib/HLSL/DxilModule.cpp
  6 ../lib/HLSL/DxilGenerationPass.cpp
  6 ../lib/HLSL/DxcOptimizer.cpp
  5 ../tools/clang/tools/dxcompiler/dxcompilerobj.cpp
  5 ../tools/clang/lib/CodeGen/CGDebugInfo.cpp
  5 ../tools/clang/lib/CodeGen/CGCall.cpp
  5 ../lib/HLSL/DxilDebugInstrumentation.cpp
  4 ../tools/clang/tools/dxcompiler/dxcfilesystem.cpp
  4 ../tools/clang/lib/SPIRV/ModuleBuilder.cpp
  4 ../tools/clang/lib/SPIRV/GlPerVertex.cpp
  4 ../tools/clang/lib/CodeGen/ItaniumCXXABI.cpp
  4 ../tools/clang/lib/CodeGen/CGExprCXX.cpp
  4 ../tools/clang/lib/AST/HlslTypes.cpp
  4 ../tools/clang/lib/AST/ExprConstant.cpp
  4 ../lib/IR/AutoUpgrade.cpp
  4 ../lib/HLSL/HLOperations.cpp
  4 ../lib/HLSL/HLOperationLower.cpp
  4 ../lib/HLSL/HLModule.cpp
  4 ../lib/HLSL/DxilMetadataHelper.cpp
  4 ../lib/HLSL/DxilContainerAssembler.cpp
  3 ../tools/clang/tools/libclang/dxcrewriteunused.cpp
  3 ../tools/clang/tools/libclang/CIndex.cpp
  3 ../tools/clang/tools/dxcompiler/dxcvalidator.cpp
  3 ../tools/clang/tools/dxcompiler/dxcontainerbuilder.cpp
  3 ../tools/clang/tools/dxcompiler/dxcassembler.cpp
  3 ../tools/clang/lib/Sema/SemaOverload.cpp
  3 ../tools/clang/lib/Sema/SemaAccess.cpp
  3 ../tools/clang/lib/CodeGen/CodeGenModule.cpp
  3 tools/clang/include/clang/AST/StmtNodes.inc
  3 ../lib/Transforms/Scalar/ScalarReplAggregatesHLSL.cpp
  3 ../lib/Transforms/IPO/PassManagerBuilder.cpp
  3 ../lib/IR/LegacyPassManager.cpp
  3 ../lib/IR/Function.cpp
  3 ../lib/IR/DiagnosticInfo.cpp
  3 ../lib/HLSL/DxilTypeSystem.cpp
  3 ../lib/HLSL/DxilShaderAccessTracking.cpp
  2 ../tools/clang/lib/Parse/HLSLRootSignature.cpp
  2 ../tools/clang/lib/CodeGen/CGStmtOpenMP.cpp
  2 ../tools/clang/lib/CodeGen/CGExprScalar.cpp
  2 ../tools/clang/lib/CodeGen/CGExprAgg.cpp
  2 ../tools/clang/lib/Basic/Targets.cpp
  2 ../tools/clang/lib/AST/MicrosoftMangle.cpp
  2 ../tools/clang/lib/AST/Decl.cpp
  2 ../tools/clang/lib/AST/ASTContext.cpp
  2 tools/clang/include/clang/AST/DeclNodes.inc
  2 ../lib/Transforms/InstCombine/InstCombineCalls.cpp
  2 ../lib/Support/CommandLine.cpp
  2 ../lib/HLSL/PauseResumePasses.cpp
  2 ../lib/HLSL/ComputeViewIdState.cpp
  2 ../lib/DxcSupport/dxcapi.use.cpp
  2 ../include/llvm/ADT/PointerUnion.h
  2 ../include/dxc/HLSL/DxilModule.h
  1 ../tools/clang/tools/libclang/CXCursor.cpp
  1 ../tools/clang/tools/dxcompiler/dxillib.cpp
  1 ../tools/clang/tools/dxcompiler/dxcutil.cpp
  1 ../tools/clang/lib/SPIRV/GlPerVertex.h
  1 ../tools/clang/lib/SPIRV/FeatureManager.cpp
  1 ../tools/clang/lib/SPIRV/DeclResultIdMapper.h
  1 ../tools/clang/lib/Sema/SemaType.cpp
  1 ../tools/clang/lib/Sema/SemaTemplateVariadic.cpp
  1 ../tools/clang/lib/Sema/SemaTemplateDeduction.cpp
  1 ../tools/clang/lib/Sema/SemaOpenMP.cpp
  1 ../tools/clang/lib/Sema/SemaExprObjC.cpp
  1 ../tools/clang/lib/Sema/SemaExpr.cpp
  1 ../tools/clang/lib/Sema/SemaExceptionSpec.cpp
  1 ../tools/clang/lib/Sema/DeclSpec.cpp
  1 ../tools/clang/lib/Parse/ParseTemplate.cpp
  1 ../tools/clang/lib/Parse/ParseStmtAsm.cpp
  1 ../tools/clang/lib/Parse/ParseDecl.cpp
  1 ../tools/clang/lib/Parse/HLSLRootSignature.h
  1 ../tools/clang/lib/Lex/TokenLexer.cpp
  1 ../tools/clang/lib/Frontend/FrontendActions.cpp
  1 ../tools/clang/lib/Frontend/FrontendAction.cpp
  1 ../tools/clang/lib/Frontend/CompilerInvocation.cpp
  1 ../tools/clang/lib/Frontend/CompilerInstance.cpp
  1 ../tools/clang/lib/CodeGen/TargetInfo.cpp
  1 ../tools/clang/lib/CodeGen/CGStmt.cpp
  1 ../tools/clang/lib/CodeGen/CGLoopInfo.cpp
  1 ../tools/clang/lib/CodeGen/CGExpr.cpp
  1 ../tools/clang/lib/CodeGen/CGExprConstant.cpp
  1 ../tools/clang/lib/CodeGen/CGExprComplex.cpp
  1 ../tools/clang/lib/CodeGen/CGDecl.cpp
  1 ../tools/clang/lib/CodeGen/CGCleanup.cpp
  1 ../tools/clang/lib/CodeGen/CGClass.cpp
  1 ../tools/clang/lib/CodeGen/BackendUtil.cpp
  1 ../tools/clang/lib/AST/TypePrinter.cpp
  1 ../tools/clang/lib/AST/ItaniumMangle.cpp
  1 ../tools/clang/lib/AST/Expr.cpp
  1 ../lib/Transforms/Utils/SimplifyCFG.cpp
  1 ../lib/Target/TargetMachine.cpp
  1 ../lib/Support/Timer.cpp
  1 ../lib/Support/GraphWriter.cpp
  1 ../lib/ProfileData/CoverageMappingReader.cpp
  1 ../lib/IR/Verifier.cpp
  1 ../lib/HLSL/ReducibilityAnalysis.cpp
  1 ../lib/HLSL/HLOperationLowerExtension.cpp
  1 ../lib/HLSL/DxilSignatureElement.cpp
  1 ../lib/HLSL/DxilSignature.cpp
  1 ../lib/HLSL/DxilOutputColorBecomesConstant.cpp
  1 ../lib/HLSL/DxilConvergent.cpp
  1 ../lib/HLSL/DxilAddPixelHitInstrumentation.cpp
  1 ../lib/Analysis/ConstantFolding.cpp
  1 ../include/dxc/HLSL/DxilSigPoint.inl