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
Files compiled with C++ for OpenCL mode can now have a distinct
file extension - clcpp, then clang driver picks the compilation
mode automatically (-x clcpp) without the use of -cl-std=clc++.
Differential Revision: https://reviews.llvm.org/D96771