Use ty_members sym_def_loc to get documentation for autocomplete

Summary:
Before this change, we looked up member location information using
`GetDef_js.extract_member_def`, whose implementation is based on the crufty
Members API.

This diff uses the Ty_members API instead, which is a more modern API based on
the type normalizer. Importantly Ty_members deals with recursive definitions
nicely in places where Members can fall into non-termination, leading to stack
overflow.

Reviewed By: panagosg7

Differential Revision: D30362536

fbshipit-source-id: e37bee0b70d7267a0839b44f6d2dc065fd5288d5

Author: samwgoldman