Improve clang-format vim integration.

The previous line-by-line replacement causes vim to take a long time if
the foldmethod is set to 'syntax'. This should significantly improve
performance in that case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186660 91177308-0d34-0410-b5e6-96231b3b80d8