[flang] Catch obscure error in defined assignment

[klausler]

Summary

A subroutine that implements a defined assignment cannot have
a dummy argument for its second operand (the RHS of the assignment)
with the POINTER or ALLOCATABLE attributes, since the RHS of
an assignment is always an expression. This problem is flagged
as a fatal error in other compilers, so let's make it fatal here
as well.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/check-declarations.cpp

if (pos == 0) {
               " must have INTENT(OUT) or INTENT(INOUT)"_err_en_US;
       }
     } else if (pos == 1) {
       if (dataObject->intent != common::Intent::In &&
           !dataObject->attrs.test(DummyDataObject::Attr::Value)) {
         msg =
             "In defined assignment subroutine '%s', second dummy"
             " argument '%s' must have INTENT(IN) or VALUE attribute"_err_en_US;
+      } else if (dataObject->attrs.test(DummyDataObject::Attr::Pointer)) {
+        msg =
+            "In defined assignment subroutine '%s', second dummy argument '%s' must not be a pointer"_err_en_US;
+      } else if (dataObject->attrs.test(DummyDataObject::Attr::Allocatable)) {
+        msg =
+            "In defined assignment subroutine '%s', second dummy argument '%s' must not be an allocatable"_err_en_US;
       }
     } else {
       DIE("pos must be 0 or 1");
     }
   } else {
     msg = "In defined assignment subroutine '%s', dummy argument '%s'"
           " must be a data object"_err_en_US;
   }

flang/test/Semantics/resolve65.f90

subroutine s2(x, y)
       import t
       type(t), intent(out) :: x
       intent(in) :: y
       interface
         subroutine y()
         end
       end interface
     end
+    !ERROR: In defined assignment subroutine 's3', second dummy argument 'y' must not be a pointer
+    subroutine s3(x, y)
+      import t
+      type(t), intent(out) :: x
+      type(t), intent(in), pointer :: y
+    end
+    !ERROR: In defined assignment subroutine 's4', second dummy argument 'y' must not be an allocatable
+    subroutine s4(x, y)
+      import t
+      type(t), intent(out) :: x
+      type(t), intent(in), allocatable :: y
+    end
   end interface
 end
 
 ! Detect defined assignment that conflicts with intrinsic assignment
 module m5
   type :: t
   end type
   interface assignment(=)