[libcxx] [llvm] [libc++] Kickstart C++29 support (PR #215820)

[Nikolas Klauser via llvm-commits]

https://github.com/philnik777 commented:

libcxx/test/tools/clang_tidy_checks/proper_version_checks.cpp will also have to be updated.

https://github.com/llvm/llvm-project/pull/215820