Enfore #must_tail can only be used at toplevel procedure scopes

[FourteenBrush]

Fixes #6640 (llvm codegen failure when using #must_tail on non toplevel proc scopes)

Tested this fix with the following code:

package main

main :: proc() {
	// some comment
	s()
	#must_tail s()
	
	call :: proc() {
		{
			s()
			#must_tail p() // error
		}
		
		#must_tail s()
		
		p :: proc() {
			
			{}
			{
				
				#must_tail s() // error
			}
		}
		for _ in 1..<3 {
			#must_tail s() // error
		}
	}
}

s :: proc() {}

Which now results in the following errors:

/home/cluster2/Projects/odin/OdinDebug/main.odin(11:15) Error: '#must_tail' can only be used at a top level procedure scope
	#must_tail p()
	           ^~^

/home/cluster2/Projects/odin/OdinDebug/main.odin(21:16) Error: '#must_tail' can only be used at a top level procedure scope
	#must_tail s()
	           ^~^

/home/cluster2/Projects/odin/OdinDebug/main.odin(25:15) Error: '#must_tail' can only be used at a top level procedure scope
	#must_tail s()

--
I intended to fix #6529 as well, but I'm not sure what the desired behavior should be of using deferred statements in combination with a #must_tail, simply disallowing it?

[antisaling]

WRT #6529 I do agree that it should emit an error in the checker like: defer cannot be used in the same scope as #must_tail (as an error on the defer call) itself. if you still think this is up to interpretation maybe ping ginger on discord or leave it for another PR