Allow diagnostic handlers to check for optimization remarks.

Summary:
When optimization remarks are enabled via the driver flag -Rpass, we
should allow the FE diagnostic handler to check if the given pass name
needs a diagnostic.

We were unconditionally checking the pattern defined in opt's
-pass-remarks flag. This was causing the FE to not emit any diagnostics.

Reviewers: qcolombet

CC: llvm-commits

Differential Revision: http://reviews.llvm.org/D3362