Pmc: Treat `x ~ y @ty` the same as `x ~ y` (#23145)

In #23145 we had a desugaring that matched on expressions ds @Any and ds @blah (where blah /= Any). In both cases, we match on the same value, but Long-distance information was unable to figure this out.

The fix is rather simple: Upon reasoning about the Core constraint x ~ ds @Any, drop any type arguments to see x ~ ds, then we will equate that to the same as ds @blah.

This plays a bit fast-and-loose with types; on the other hand, the situations in which can give the same value different types should be exceedingly rare.

Fixes #23145.