Basic support for diagnostics.

Summary: Uses DiagnosticsEngine to output diagnostics.

Reviewers: djasper, klimek

Reviewed By: djasper

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D278

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172071 91177308-0d34-0410-b5e6-96231b3b80d8