Add support for raw_ostream on the printing methods of Diagnostics.

Summary:
Add printToStream*(llvm::raw_ostream&) methods to Diagnostics, and reimplement everything based on streams instead of concatenating strings.
Also, fix some functions to start with lowercase to match the style guide.

Reviewers: klimek

CC: cfe-commits, revane

Differential Revision: http://llvm-reviews.chandlerc.com/D1187