[clang-format] append newline after code when inserting new headers at the end...

[clang-format] append newline after code when inserting new headers at the end of the code which does not end with newline.

Summary:
append newline after code when inserting new headers at the end of the
code which does not end with newline.

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D21026

llvm-svn: 283330