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

llvm-svn: 197138