Initial check in of Doug's modularize tool for checking header readiness for modules, plus some changes per review feedback:  Removed system headers, changed to use stream output, added file name in message.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@176847 91177308-0d34-0410-b5e6-96231b3b80d8
[docs] Clarify wording.

"This documentation" could be construed as the entire c-t-e docs
(despite being under a doxygen heading; people read "locally"
sometimes), which gives a bad impression.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@176849 91177308-0d34-0410-b5e6-96231b3b80d8