TR: Prevent internal error on `begin-for-syntax`

Closes PR 13878

Please merge to v6.0
Caveat section on macros/phase-1+ code in TR

Please merge to v6.0
Improve type of `negate`

Now for functions with simple filters, it'll negate the
filters so that, e.g., (negate string?) can be used with
occurrence typing.

Please merge to v6.0