[Sema] isValidCoroutineContext FIXME and citations

Summary:
Add citations to the Coroutines TS to the `isValidCoroutineContext`
function, as well as a FIXME and test for [expr.await]p2, which states
a co_await expression cannot be used in a default argument.

Test Plan: check-clang

Reviewers: GorNishanov, EricWF

Reviewed By: GorNishanov

Subscribers: rsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D48519

llvm-svn: 335420