[clang] [libcxx] [Clang] Add __builtin_invoke and use it in libc++ (PR #116709)

[via cfe-commits]

higher-performance wrote:

Commenting here for anyone interested: in #213403 I have a large speed improvement that makes the common case (ordinary function/non-member calls) _faster_ than the compiler built-in. I was thinking it could even be motivation to remove `__builtin_invoke` and avoid its maintenance burden, but not sure what folks think.

https://github.com/llvm/llvm-project/pull/116709