Submitted by: Bill Wendling

Reviewed by: Chris Lattner

- Added a new diagnostic client, TextDiagnosticBuffer. It buffers all
  reported diagnostics.
- Use the new diagnostic client to check that expected diagnostics are
  actually emitted. The way this is done is to put the expected
  diagnostic in a comment on the line you expect it to be emitted for.
  Like this:

    int X = A; // expected-warning {{blah}}

- Use -parse-ast-check to use this feature.

llvm-svn: 39678