[OpenCL] Reduce emitting candidate notes for builtins

When overload resolution fails, clang emits a note diagnostic for each
candidate.  For OpenCL builtins this often leads to many repeated note
diagnostics with no new information.  Stop emitting such notes.

Update a test that was relying on counting those notes to check how
many builtins are available for certain extension configurations.

Differential Revision: https://reviews.llvm.org/D127961