[flang] Accept defined assignment with CLASS(*) RHS

[klausler]

Summary

A utility predicate in semantics was incorrectly determining that
an INTERFACE ASSIGNMENT(=) (or other form of generic) could not have
a specific procedure with an unlimited polymorphic second argument.
This led to a crash later in expression analysis. Fix, and
extend tests.

flang/lib/Semantics/tools.cpp

 }
 
 Tristate IsDefinedAssignment(
     const std::optional<evaluate::DynamicType> &lhsType, int lhsRank,
     const std::optional<evaluate::DynamicType> &rhsType, int rhsRank) {
   if (!lhsType || !rhsType) {
     return Tristate::No; // error or rhs is untyped
   }
-  if (lhsType->IsUnlimitedPolymorphic() || rhsType->IsUnlimitedPolymorphic()) {
+  if (lhsType->IsUnlimitedPolymorphic()) {
     return Tristate::No;
   }
+  if (rhsType->IsUnlimitedPolymorphic()) {
+    return Tristate::Maybe;
+  }
   TypeCategory lhsCat{lhsType->category()};
   TypeCategory rhsCat{rhsType->category()};
   if (rhsRank > 0 && lhsRank != rhsRank) {
     return Tristate::Yes;
   } else if (lhsCat != TypeCategory::Derived) {
     return ToTristate(lhsCat != rhsCat &&
         (!IsNumericTypeCategory(lhsCat) || !IsNumericTypeCategory(rhsCat)));
   } else if (MightBeSameDerivedType(lhsType, rhsType)) {

flang/test/Semantics/defined-ops.f90

module m3
   type t
   end type
   interface assignment(=)
     subroutine s1(x, y)
       import
       class(t), intent(out) :: x
       integer, intent(in) :: y
     end
+    subroutine s2(x, y)
+      real, intent(out) :: x
+      class(*), intent(in) :: y
+    end
   end interface
   interface operator(+)
     integer function f(x, y)
       import
       class(t), intent(in) :: x, y
     end
   end interface
 contains
-  subroutine test(x, y)
+  subroutine test(x, y, z)
     class(t) :: x, y
+    class(*), intent(in) :: z
+    real :: a
     !CHECK: CALL s1(x,2_4)
     x = 2
     !CHECK: i=f(x,y)
     i = x + y
+    !CHECK: CALL s2(a,z)
+    a = z
   end
 end
-