SI-6951 Add test to show we aren't missing .tpeHK

We're getting the inferred type arguments to unapply
from the a tree that we decompose with `Applied`,
rather than from symbols. As such, the call to .tpe
is correct. In the additional test case, the substition
of `M => Array` works.
SI-6951 Allow for checkable type without base classes

If we ignore an unchecked warning and proceed into the
pattern matcher, the checkable type might be completely
abstract, and as such have no base classes. Use `Any`
in that case.

This wasn't the reported bug, but something I found while
testing its fix.