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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196928 91177308-0d34-0410-b5e6-96231b3b80d8