SI-7967 Account for type aliases in self-type checks

These eluded the check for "illegal inheritance; self-type does not
conform" as AliasTypeRef doesn't forward `typeOfThis`.

This commit dealiases before calling `typeOfThis`. That seems to be
the most localised change.

Without the dealias, we had:

    parent.tpe            = TypeRef(pre, sym = AliasTypeSymbol("CC"), Nil)
    parent.tpe.typeOfThis = parent.tpe.transform(sym.typeOfThis) = CC

After:

    parent.tpe.dealias            = TypeRef(pre, sym = ClassSymbol("C"), Nil)
    parent.tpe.dealias.typeOfThis = C with B

Review by @adriaanm