[OpenCL] Relax diagnostics on OpenCL access qualifiers

Summary:
Emit warning for multiple access qualifiers if they do not conflict.

Patch by Alexey Bader

Reviewers: Anastasia, yaxunl

Reviewed By: Anastasia

Subscribers: asavonic, bader, cfe-commits

Differential Revision: https://reviews.llvm.org/D51302

llvm-svn: 341553