[flang] Reverse a reversed type compatibility check

The semantic test for an intrinsic assignment to a polymorphic
derived type entity from a type that is an extension of its base
type was reversed, so it would allow assignments that it shouldn't
and disallowed some that it should; and the test case for it
incorectly assumed that the invalid semantics were correct.
Fix the code and the test, and add a new test for the invalid
case (LHS type is an extension of the RHS type).

Differential Revision: https://reviews.llvm.org/D125135