[clang] Adjust coroutine namespace diagnostics

The diagnostics concerning mixing std::experimental and std are
somewhat wordy and have some typographical errors.  Diagnostics do not
start with a capital letter nor end with a fullstop.  Usually we try
and link clauses with a semicolon, rather than start a new sentence.
So that's what this patch does.  Along with avoiding repetition about
std::experimental going away.

Differential Revision: https://reviews.llvm.org/D116026