Let clang-format format itself.

Apply all formatting changes that clang-format would apply to its own source
code. All choices seem to improve readability (or at least not make it worse).
No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171039 91177308-0d34-0410-b5e6-96231b3b80d8