[Coroutines] Catch exceptions in await_resume

Summary:
http://wg21.link/P0664r2 section "Evolution/Core Issues 24" describes a
proposed change to Coroutines TS that would have any exceptions thrown
after the initial suspend point of a coroutine be caught by the handler
specified by the promise type's 'unhandled_exception' member function.
This commit provides a sample implementation of the specified behavior.

Test Plan: `check-clang`

Reviewers: GorNishanov, EricWF

Reviewed By: GorNishanov

Subscribers: cfe-commits, lewissbaker, eric_niebler

Differential Revision: https://reviews.llvm.org/D45860

llvm-svn: 331519