Draft: Fix unusable unpack warning on UnliftedType

I fixed a wrongly emitted warning regarding the UNPACK pragma. I discovered it in relation to #20204.

In that issue there was the problem that data Test = TestCon {-# UNPACK #-} IntU wrongly claimed that a bang was required to use the pragma. However, IntU was an UnliftedType and therefore the bang does not make sense. This problem has been fixed but another related issue came up. Namely, GHC currently reports the UNPACK as being unusable, but in the case of IntU that is not the case, because (IIRC) IntU is still a box, it just always contains a value. Therefore the UNPACK removing a level of indirection is certainly valid. I fixed this by adding a check that if HsLazy-types (i.e. they're was no bang) are unlifted and not primitive, then the UNPACK still make sense.

The not primitive part is important such that {-# UNPACK #-} Int# is still reported as unusable. I am only mildly certain that that predicate is precise enough. (Let's see what the testsuite says)

I also added the described situations in two extra tests.

I do believe that if this change is accepted, #20204 has been resolved entirely.