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

[Louis Dionne via cfe-commits]

https://github.com/ldionne approved this pull request.

I'm fine with the libc++ part of this patch. I like this new approach a lot more. I think it gives us most of the benefits without raising difficult questions.

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