Implemented GNU-style formatting for compound statements.

Summary:
Added BraceBreakingStyle::BS_GNU. I'm not sure about the correctness of
static initializer formatting, but compound statements should be fine.

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D2372

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197138 91177308-0d34-0410-b5e6-96231b3b80d8