Sort all the #include lines with LLVM's utils/sort_includes.py which
encodes the canonical rules for LLVM's style. I noticed this had drifted
quite a bit when cleaning up LLVM, so wanted to clean up Clang as well.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@198686 91177308-0d34-0410-b5e6-96231b3b80d8