[SEMA]Generalize deferred diagnostic interface, NFC.

Summary:
Deferred diagnostic interface is going to be used for OpenMP device
compilation. Generalized previously existed deferred diagnostic
interface for CUDA to be used with OpenMP and, possibly, other models.

Reviewers: rjmccall, tra

Subscribers: caomhin, cfe-commits, kkwli0

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57908

llvm-svn: 353456