[coroutines] Diagnose invalid result types for `await_resume` and `await_suspend...

[coroutines] Diagnose invalid result  types for `await_resume` and `await_suspend` and add missing conversions.

Summary:
The expression `await_ready` is required to be contextually convertible to bool and `await_suspend` must be a prvalue of either `void` or `bool`.
This patch adds diagnostics for when those requirements are violated.

It also correctly performs the contextual conversion to bool on the result of `await_ready`

Reviewers: GorNishanov, rsmith

Reviewed By: GorNishanov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D33625

llvm-svn: 304094