Revert "Handle diagnostic warnings in Frontend diagnostic handler."

This commit is missing tests and there are a few points that need to be
addressed before a new user-facing option can be added:

  http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20140714/110198.html

This reverts commit r213112.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213260 91177308-0d34-0410-b5e6-96231b3b80d8