Update clang-format emacs integration.

- Remove free variables
- Add function clang-format-buffer, e.g. for before-save-hooks
- Wrap restoring windows in an unwind-protect

Patch by Stephen Gildea!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181766 91177308-0d34-0410-b5e6-96231b3b80d8