[clang-tidy] Add a little python script that can run clang-tidy and apply fixes over an entire codebase.

Ever wanted to fix all the header guards in clang? Now it's easy.
  Make sure clang-tidy is in $PATH and a compilation database is available.
  $ ./run-clang-tidy.py -checks=-*,llvm-header-guard -fix
  ... get coffee (or more CPU cores) ...
  $ svn diff

Some may argue that this is just a glorified xargs -P, but it does a bit more ;)

Differential Revision: http://reviews.llvm.org/D5188