[coroutines] Fix application of NRVO to Coroutine "Gro" or return object.

Summary:
Fix NRVO for Gro variable.

Previously, we only marked the GRO declaration as an NRVO variable
when its QualType and the function return's QualType matched exactly
(using operator==). However, this was incorrect for two reasons:

1. We were marking non-class types, such as ints, as being NRVO variables.

2. We failed to  handle cases where the canonical types were the same, but the actual `QualType` objects were different. For example, if  one was represented by a typedef. (Example: https://godbolt.org/g/3UFgsL)

This patch fixes these bugs by marking the Gro variable as supporting NRVO only
when `BuildReturnStmt` marks the Gro variable as a coroutine candidate.






Reviewers: rsmith, GorNishanov, nicholas

Reviewed By: GorNishanov

Subscribers: majnemer, cfe-commits

Differential Revision: https://reviews.llvm.org/D42343

llvm-svn: 324037