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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@130492 91177308-0d34-0410-b5e6-96231b3b80d8