Factored out part of `conv_object_ty_poly_trait_ref` method.
Factored out part of `conv_object_ty_poly_trait_ref` method.

aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust