[clang-format] skip empty lines and comments in the top of the code when inserting new headers.

Summary:
[clang-format] skip empty lines and comments in the top of the code when inserting new headers.

Pair-programmed with @hokein

Reviewers: djasper

Subscribers: ioeric, cfe-commits, hokein, klimek

Differential Revision: http://reviews.llvm.org/D20898

llvm-svn: 271664