Give the "cannot pass object of non-POD type 'class C' through variadic constructor...

Give the "cannot pass object of non-POD type 'class C' through variadic constructor; call will abort at runtime" warning a -W flag (non-pod-varargs) and default it being an error by default.  There is no good reason to allow users to get bitten by this sort of thing by default.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@91094 91177308-0d34-0410-b5e6-96231b3b80d8