Check with Mypyc level of strictness #15757

Pitch

There's regular Mypy, and then there's strict Mypy. But even stricter than that is Mypyc. Mypyc is strictly stricter than strict Mypy -- everything accepted by Mypyc is accepted by strict Mypy, but not vice versa.

Currently, checking with the Mypyc level of strictness requires actually running Mypyc, and that means doing the code generation, which is slow.

It would be nice to be able to check with Mypyc level of strictness without doing all the code generation. Something along the lines of mypy --super-strict. Or maybe mypyc --check.