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

[Erich Keane via cfe-commits]

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

I'm happy as-is.  Please give Aaron a chance to poke if he wants to.

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