Address review comments from Matt on the sort_includes.py script.

Address review comments from Matt on the sort_includes.py script.

1) Teach it to handle files with #include on the first line -- these do
   actually exist in LLVM.
2) Support llvm-c and clang-c include projects.
3) Nuke some stail imports.
4) Switch to using os.path to split the file extension off.
5) Remove debugging leftovers.
6) Add docstring (a really puny one) for the sort function.

I'm continuing te avoid stripping the whitespace on the RHS to preserve
whatever newline characters happen to be in the original file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169222 91177308-0d34-0410-b5e6-96231b3b80d8