[OpenCL] Fix diagnostic message about overload candidates

Summary:
I wonder if there are some extension which need to be disabled to get
overloadable candidate available.


Reviewers: asavonic, Anastasia

Reviewed By: Anastasia

Subscribers: yaxunl, sidorovd, cfe-commits

Differential Revision: https://reviews.llvm.org/D54152

llvm-svn: 346311