[clang-format] Preserve include blocks in ObjC Google style

Summary:
r357567 started to regroup include block for Google style; it was meant to apply
only for C++. This patch reverts this for ObjC.

Reviewers: ioeric

Reviewed By: ioeric

Subscribers: thakis, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60263

llvm-svn: 357695