move resolve_attribute_access out of forward_expression

Summary:
The diff view is pretty confusing, but this diff just lifts resolve_attribute_access out of forward_expression into the top level.

I think we want to call this (or some subset of it) during augmented assignments  to handle the fallback from `__i*__` to `__*__`

Reviewed By: samwgoldman

Differential Revision: D58415997

fbshipit-source-id: 2d33d5ce1107ea0752c4077ac8eb2f7ca607eb71