[coroutines] Add support for symmetric control transfer (musttail on...

[coroutines] Add support for symmetric control transfer (musttail on coro.resumes followed by a suspend)

Summary:
Add musttail to any resume instructions that is immediately followed by a
suspend (i.e. ret). We do this even in -O0 to support guaranteed tail call
for symmetrical coroutine control transfer (C++ Coroutines TS extension).
This transformation is done only in the resume part of the coroutine that has
identical signature and calling convention as the coro.resume call.

Reviewers: GorNishanov

Reviewed By: GorNishanov

Subscribers: EricWF, majnemer, llvm-commits

Differential Revision: https://reviews.llvm.org/D37125

llvm-svn: 311751