Add -pass-remarks flag to 'opt'.

Summary:
This adds support in 'opt' to filter pass remarks emitted by
optimization passes. A new flag -pass-remarks specifies which
passes should emit a diagnostic when LLVMContext::emitOptimizationRemark
is invoked.

This will allow the front end to simply pass along the regular
expression from its own -Rpass flag when launching the backend.

Depends on D3227.

Reviewers: qcolombet

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3291

llvm-svn: 205775