[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

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@217368 91177308-0d34-0410-b5e6-96231b3b80d8
Move clang-tools-extra docs 1.5 years into the future.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@217369 91177308-0d34-0410-b5e6-96231b3b80d8