Fix type directed disambiguation of optional arg defaults

[ncik-roberts]

Fix the specific issue in #12668.

The approach: In type-checking, I duplicate the type annotation on an optional argument's Ppat_constraint on the optional argument default expression. This is a bit of a hack but it is non-invasive.

I'm not confident that this approach fixes all potential backward-incompatibilities from #12236 (as optional argument defaults used to be type-checked more like let than like match) but I can't find another case where it matters.

[Octachron]

This change was discussed at the type system meetings today, and it seems consistent with the let-binding behavior and a good step forward.

[Octachron]

@ncik-roberts , the Changes entry needs to be updated before merging. Do you want me to take care of it?

[ncik-roberts]

Looking now.

[ncik-roberts]

Should be fixed now.

[Octachron]

Thanks!