Clean up the "non-POD memaccess" stuff some. This adds a properly named

diagnostic group to cover the cases where we have definitively bad
behavior: dynamic classes.

It also rips out the existing support for POD-based checking. This
didn't work well, and triggered too many false positives. I'm looking
into a possibly more principled way to warn on the fundamental buggy
construct here. POD-ness isn't the critical aspect anyways, so a clean
slate is better. This also removes some silliness from the code until
the new checks arrive.

llvm-svn: 132534