[OpenCL] Extended diagnostics for atomic initialization

Summary:
I saw the same changes in the following review: https://reviews.llvm.org/D17438

I don't know in that way I could determine that atomic variable was initialized by macro ATOMIC_VAR_INIT. Anyway I added check that atomic variables can be initialize only in global scope.
I think that we can discuss this change.

Reviewers: Anastasia, cfe-commits

Reviewed By: Anastasia

Subscribers: bader, yaxunl

Differential Revision: https://reviews.llvm.org/D30643

llvm-svn: 299537