[clang] [Clang][Sema] Backport P2741R3 (static_assert with user-generated message) to C++11 (PR #102044)

[Mital Ashok via cfe-commits]

MitalAshok wrote:

This doesn't make sense to backport to C++98's `_Static_assert` extension (no `constexpr` member functions)

CC @cor3ntin @AaronBallman 

Re: affects on libc++/any downstream:
Other than the feature test macro, this extension should not affect any valid C++11/20 program, since previously it just didn't parse.


https://github.com/llvm/llvm-project/pull/102044