Use DiagnosticRenderer to convert clang diagnostics to clang-tidy ones.

Summary:
This is immediately useful for generating macro expansion notes, and
may be useful for other things later on.

Reviewers: klimek, djasper

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2950

llvm-svn: 203457