[clang-format] insert new #includes into correct blocks when cleaning up...

[clang-format] insert new #includes into correct blocks when cleaning up Replacement with cleanupAroundReplacements().

Summary:
When a replacement's offset is set to UINT_MAX or -1U, it is treated as
a header insertion replacement by cleanupAroundReplacements(). The new #include
directive is then inserted into the correct block.

Reviewers: klimek, djasper

Subscribers: klimek, cfe-commits, bkramer

Differential Revision: http://reviews.llvm.org/D20734

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271276 91177308-0d34-0410-b5e6-96231b3b80d8