Relax the non-POD memset warning to use the less restrictive C++11

definition of POD. Specifically, this allows certain non-aggregate
types due to their data members being private.

The representation of C++11 POD testing is pretty gross. Any suggestions
for improvements there are welcome. Especially the name
'isCXX11PODType()' seems truly unfortunate.

llvm-svn: 130492