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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Couple of nits, else I'm reasonably ok with this.  Interested to hear what @AaronBallman has to say, else I think this is beneficial.

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