Fix llvm-header-guard check.

Summary:
This patch makes the check work better for LLVM code:
  * always fix existing #endif comments
  * use one space before the comment (+allow customization for other styles)

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5795

llvm-svn: 219789