Handle assertion error in TyperState (scala#23665)

scala#23609 triggers an assertion error in TyperState. The relevant
explanation seems to be in ProtoTypes.scala:
```scala
            // To respect the pre-condition of `mergeConstraintWith` and keep
            // `protoTyperState` committable we must ensure that it does not
            // contain any type variable which don't already exist in the passed
            // TyperState. This is achieved by instantiating any such type
            // variable. NOTE: this does not suffice to discard type variables
            // in ancestors of `protoTyperState`, if this situation ever
            // comes up, an assertion in TyperState will trigger and this code
            // will need to be generalized.
```
We should go to the bottom of it and fix the assertion. But before
that's done this PR offers a temporary hack to catch the exception when
it is triggered from a new code path created by PR scala#23532. This should
fix the regression reported in scala#23609. We should leave the issue open as
a reminder that we still need a better fix.

Also: handle crash due to missing span in a migration helper.
[Cherry-picked 408298d][modified]

Author: tgodzik