[Sema][OpenCL] Improve diagnostics for not viable overloadable function candidates

Summary:
Allowed extension name (that ought to be disabled) printing in the note message.

This diagnostic was proposed here: https://reviews.llvm.org/D51341

Reviewers: Anastasia, yaxunl

Reviewed By: Anastasia

Subscribers: cfe-commits, asavonic, bader

Differential Revision: https://reviews.llvm.org/D52292

llvm-svn: 344246