Disable -Wnon-pod-memset for now while I try to reduce the false

positives still further.

The plan is to:

1) Create a more targeted warning for memset of memory pointing at
   a type with virtual methods or bases where a vptr would be
   overwritten.
2) Consider turning the above warning back on by default.
3) Evaluate whether any false positives in the existing warning can be
   detected and white listed in the warning implementation.
4) If #3 lowers the noise floor enough, enable the full warning in -Wall
   or -Wextra.

Comments or suggestions welcome. Even more welcome are specific test
cases which trigger the warning and shouldn't.

llvm-svn: 130538