clang-format: Smarter replacement in the vim integration.

With this fix, only changed regions will be replaced in vim's buffer.
Thereby, marks should mostly be left intact. Furthermore, this is a
better fix for the performance problem in conjunction with
'foldmethod=syntax' (see r186660).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186789 91177308-0d34-0410-b5e6-96231b3b80d8