ObjC++: decorate ObjC interfaces in MSABI properly

`id` needs to be handled specially since it is a `TypedefType` which is
sugar for an `ObjCObjectPointerType` whose pointee is an
`ObjCObjectType` with base `BuiltinType::ObjCIdType` and no protocols
and the first level of pointer gets it own type implementation.  `Class`
is similar with the `ObjCClassType` as the base instead.

The qualifiers on the base type of the `ObjCObjectType` need to be
dropped because the innermost `mangleType` will handle the qualifiers
itself.

`id` is desugared to `struct objc_object *` which should be encoded as
`PAUobjc_object@@`.  `Class` is desugared to `struct objc_class *` which
should be encoded as `PAUobjc_class@@`.

We were previously applying an extra modifier `A` which will be handled
during the recursive call.

This now properly decorates interface types as well as `Class` and `id`.
This corrects the interactions between C++ and ObjC++ for the type
specifier decoration.

llvm-svn: 311617