Correctly instantiate generic attributes for XHP
Summary:
We weren't properly instantiating the `this` type or generics for XHP literals
because we fetched the type directly from `class_type.tc_prop`. There is a lot
of logic involved in properly resolving these types, so the easiest thing to do
was to get the types of the individual attributes via `Typing.obj_get` to make
sure expression dependent types and generics are instantiated correctly.
Reviewed By: andrewjkennedy
Differential Revision:
D3375690
fbshipit-source-id:
76f160ff39a56bbf3a9822bdb5ce0b79421c9204