Remove LLVMContextImpl::optimizationRemarkEnabledFor.

Summary:
This patch moves the handling of -pass-remarks* over to
lib/DiagnosticInfo.cpp. This allows the removal of the
optimizationRemarkEnabledFor functions from LLVMContextImpl, as they're
not needed anymore.

Reviewers: qcolombet

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3878

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209453 91177308-0d34-0410-b5e6-96231b3b80d8