SI-7984 Issue unchecked warning for type aliases

The unchecked nature of the type test was already noticed,
but because the type alias tripped up in rendering a sensible
message `CheckabilityChecker` chose to remain silent, other
than under `-Xdev.`

This commit:

 - Dealiases pattern types before launching the CheckabilityChecker
 - Sharpens the error messages to explain that the dealiased type
   is the expansion of the alias.