[CI] Switch testing to LLVM 20 (#374) #2346

Job	Run time
python-lint	5s
clang-format	25s
ubuntu-gcc-build	1m 27s
ubuntu-clang-build	8m 15s
macos-clang-build	2m 32s
windows-msvc-build	4m 11s
windows-clang-build	4m 39s
ubuntu-gcc-test	4m 30s
ubuntu-clang-sanitizer-build	3m 5s
ubuntu-clang-test	4m 35s
macos-clang-test	2m 47s
windows-msvc-test	2m 24s
windows-clang-test	1m 28s
ubuntu-gcc-test-extended	4m 30s
ubuntu-clang-sanitizer-test	3m 5s
macos-clang-test-extended	2m 49s
windows-msvc-test-extended	2m 20s
ubuntu-clang-test-extended	4m 46s
windows-clang-test-extended	2m 20s
ubuntu-clang-sanitizer-test-extended	2m 45s
ubuntu-gcc-build-codecov	3m 59s
ubuntu-gcc-build-perf-stats	2m 37s
	1h 9m 34s

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