Re-infer statements that got more specific on subsequent pass

Re-infer statements that got more specific on subsequent pass

Fixed issue where mypy plugin would not correctly interpret an explicit
:class:`_orm.Mapped` annotation in conjunction with a
:func:`_orm.relationship` that refers to a class by string name; the
correct annotation would be downgraded to a less specific one leading to
typing errors.

The thing figured out here is that after we've already scanned
a class in the semanal stage and created DeclClassApplied,
when we are called again with that same DeclClassApplied, for this
specific kind of case we actually now have *better* types than
we did before, where the left side that looked like
List?[Address?] now seems to say
builtins.list[official.module.Address] - so let's take the
right side expression again, this time embedded in our
Mapped._empty_constructor() expression, and run the infer
all over again just like mypy would.   Just not setting the
"wrong" type here fixed the test cases but by re-applying the
whole infer we get the correct Mapped[] on the left side too.

Fixes: #6255
Change-Id: Iafe7254374f685a8458c7a1db82aafc2ed6d0232