[CI] Switch testing to LLVM 20 #2312

Job	Run time
python-lint	6s
clang-format	9s
ubuntu-clang-build	8m 6s
ubuntu-gcc-build	1m 48s
macos-clang-build	2m 28s
windows-msvc-build	4m 4s
windows-clang-build	4m 18s
ubuntu-gcc-test	4m 29s
ubuntu-clang-test	4m 42s
ubuntu-clang-sanitizer-build	3m 4s
macos-clang-test	2m 49s
windows-clang-test	1m 25s
windows-msvc-test	2m 19s
ubuntu-gcc-test-extended	4m 26s
macos-clang-test-extended	2m 47s
ubuntu-clang-test-extended	4m 46s
ubuntu-clang-sanitizer-test	1m 57s
windows-clang-test-extended	2m 12s
windows-msvc-test-extended	2m 24s
ubuntu-gcc-build-codecov	3m 56s
ubuntu-clang-sanitizer-test-extended	2m 48s
ubuntu-gcc-build-perf-stats	1m 6s
	1h 6m 9s

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