[libcxx] [Coroutines] Support noop_coroutine for GCC

We didn't support noop_coroutine for GCC in previous conforming patch.
So that GCC couldn't use noop_coroutine() defined in <coroutine>. And
after this patch, GCC should be able to compile the whole <coroutine>
header.

Reviewed By: Quuxplusone

Differential Revision: https://reviews.llvm.org/D116144