Make imprecise constraints handling more robust (#16502)
Fixes https://github.com/python/mypy/issues/16485
My initial implementation of imprecise constraints fallback was really
fragile and ad-hoc, and I now see several edge case scenarios where we
may end up using imprecise constraints for a `ParamSpec` while some
precise ones are available. So I re-organized it: now we just infer
everything as normally, and filter out imprecise (if needed) at the very
end, when we have the full picture. I also fix an accidental omission in
`expand_type()`.
