Fixes python#13066

During the semanal phase, mypy opts to ignore and skip processing
any malformed or illegal statements inside of a TypedDict class
definition, such as method definitions.

Skipping semanal analysis on these statements can cause any number
of odd downstream problems: the type-checking phase assumes that all
semanal-only semantic constructs (e.g. FakeInfo) have been purged
by this point, and so can crash at any point once this precondition has
been violated.

This diff opts to solve this problem by filtering down the list of
statements so we keep only the ones we know are legal within a TypedDict
definition.

The other possible solution to this problem is to modify mypy so we
skip checking TypedDict class bodies entirely during type checking and
fine-grained deps analysis. Doing this would also let address python#10007
and supersede my other diff python#13732.

I decided against doing this for now because:

1. I wasn't sure if this was actually safe, especially in the
   fine-grained deps phase and for mypyc.
2. I think no matter what, the semanal phase should not leak
   semanal-only types: relaxing this postcondition would make
   it harder to reason about mypy. So, we'd probably want to make this
   change regardless of what we do in the later phases.