During the ISO C++ Committee meeting plenary session the C++23 Standard
has been voted as technical complete.
This updates the reference to c++2b to c++23 and updates the __cplusplus
macro.
Drive-by fixes c++1z -> c++17 and c++2a -> c++20 when seen.
Reviewed By: aaron.ballman
Differential Revision: https://reviews.llvm.org/D149553
Closing https://github.com/llvm/llvm-project/issues/56803. The root
cause for this bug is that we lack a good method to detect the language
mdoe when parsing the command line. There is a FIXME too. Dut to we lack
a good solution now, keep the workaround.