[CI] Switch testing to LLVM 20 (#374) #2328

Job	Run time
python-lint	11s
clang-format	17s
ubuntu-gcc-build	1m 33s
ubuntu-clang-build	9m 0s
macos-clang-build	3m 29s
windows-msvc-build	4m 8s
windows-clang-build	4m 13s
ubuntu-gcc-test	4m 23s
ubuntu-clang-sanitizer-build	3m 3s
ubuntu-clang-test	4m 35s
macos-clang-test	2m 51s
windows-msvc-test	2m 27s
windows-clang-test	1m 18s
ubuntu-gcc-test-extended	4m 27s
windows-msvc-test-extended	2m 29s
macos-clang-test-extended	3m 11s
ubuntu-clang-test-extended	4m 45s
ubuntu-clang-sanitizer-test	1m 55s
windows-clang-test-extended	2m 17s
ubuntu-clang-sanitizer-test-extended	2m 48s
ubuntu-gcc-build-codecov	5m 18s
ubuntu-gcc-build-perf-stats	2m 16s
	1h 10m 54s

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