[clang-format] Regroup #includes into blocks for Google style

Summary:
Regrouping #includes in blocks separated by blank lines when sorting C++ #include
headers was implemented recently, and it has been preferred in Google's C++ style guide:
https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes

Reviewers: sammccall, klimek

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60116

llvm-svn: 357567