[coroutines] Support coroutine-handle returning await-suspend (i.e symmetric control transfer)

Summary:
If await_suspend returns a coroutine_handle, as in the example below:
```
  coroutine_handle<> await_suspend(coroutine_handle<> h) {
    coro.promise().waiter = h;
    return coro;
  }
```
suspensionExpression processing will resume the coroutine pointed at by that handle.
Related LLVM change rL311751 makes resume calls of this kind `musttail` at any optimization level.

This enables unlimited symmetric control transfer from coroutine to coroutine without blowing up the stack.

Reviewers: GorNishanov

Reviewed By: GorNishanov

Subscribers: rsmith, EricWF, cfe-commits

Differential Revision: https://reviews.llvm.org/D37131

llvm-svn: 311762