Early attempts to format in GNU style.

Summary:
This still misses a few important features, so there's no mention of
this style in the help message, but a few style rules are implemented.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D2371

llvm-svn: 196928