PODness and Type Traits

Make C++ classes track the POD property (C++ [class]p4)
Track the existence of a copy assignment operator.
Implicitly declare the copy assignment operator if none is provided.
Implement most of the parsing job for the G++ type traits extension.
Fully implement the low-hanging fruit of the type traits:
__is_pod: Whether a type is a POD.
__is_class: Whether a type is a (non-union) class.
__is_union: Whether a type is a union.
__is_enum: Whether a type is an enum.
__is_polymorphic: Whether a type is polymorphic (C++ [class.virtual]p1).

llvm-svn: 61746