Support 'remark' in VerifyDiagnosticConsumer

After Diego added support for -Rpass=inliner we have now in-tree remarks which
we can use to properly test this feature.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207765 91177308-0d34-0410-b5e6-96231b3b80d8