[PATCH] D40144: Implement `std::launder`

[Tim Song via Phabricator via cfe-commits]

tcanens added a comment.

At least for GCC, it should use `__builtin_launder`.

Also needs to implement "The program is ill-formed if `T` is a function type or //cv// `void`."


https://reviews.llvm.org/D40144