Add a warning (-Wnon-pod-memset) for calls to memset() with

a destination pointer that points to a non-POD type. This can flag such
horrible bugs as overwriting vptrs when a previously POD structure is
suddenly given a virtual method, or creating objects that crash on
practically any use by zero-ing out a member when its changed from
a const char* to a std::string, etc.

llvm-svn: 130299