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
Since coroutine is merged in C++ standard and the support for coroutine
seems relatively stable. It's the time to move the implementation of
coroutine out of the experimental directory and the std::experimental
namespace. This patch creates header <coroutine> with conformed
implementation with C++ standard. To avoid breaking user's code too
fast, the <experimental/coroutine> header is remained. Note that
<experimental/coroutine> is deprecated and it would be removed in
LLVM15.
Reviewed By: Quuxplusone, ldionne
Differential Revision: https://reviews.llvm.org/D109433
Chuanqi Xu