Use Hint_placeholder for assigmments in name-def
Use Hint_placeholder for assigmments in name-def

Summary:
Since there is already a TODO around this, let's avoid propagating an `AnyT` here.
This came up as a way to avoid having to deal with `unclear-type` errors caused due
to processing this `AnyT` later on in the env-resolution.

Changelog: [internal]

Reviewed By: SamChou19815

Differential Revision: D38811414

fbshipit-source-id: dc4d19f854eebff9cdd7c578022a861726abfd50

mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow