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

[Hristo Hristov via llvm-commits]

Zingam wrote:

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

I believe, the necessary changes are done.

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