[coroutines] Add diagnostics for copy/move assignment operators and functions...

[coroutines] Add diagnostics for copy/move assignment operators and functions with deduced return types.

Summary: The title says it all. Additionally this patch refactors the diagnostic code into a separate function.

Reviewers: GorNishanov, rsmith

Subscribers: majnemer, mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D25292

llvm-svn: 285331