Fix `make-predicate` at the top-level

This solves the immediate problem, but more generally the
handling of ignored syntax at the top-level seems very fishy.
There is a complicated protocol involving the top-level
type-checker returning #<void> for cases with no obvious
return type, but it's unclear if it actually works. That
code in tc-toplevel.rkt should be revamped in the future.

Closes PR 14030

Please merge into 6.0