clang-tidy: Add initial check for "Don't use else after return".

As per the LLVM coding standards:
http://llvm.org/docs/CodingStandards.html#don-t-use-else-after-a-return

Initial version, probably still quite a bit to do until this is really
useful.

Review: http://reviews.llvm.org/D6927
llvm-svn: 226025