[flang] Improve error message for attempted assignment to a procedure

[klausler]

Summary

Emit a "Assignment to procedure 'foo' is not allowed" error message
for more cases of 'foo' than just declaraed subprograms, rather than
assuming that those additional cases were named constants.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/expression.cpp

     if (!symbol) {
       context_.SayAt(x, "Assignment to constant '%s' is not allowed"_err_en_US,
           x.GetSource());
-    } else if (auto *subp{symbol->detailsIf<semantics::SubprogramDetails>()}) {
-      auto *msg{context_.SayAt(x,
-          "Assignment to subprogram '%s' is not allowed"_err_en_US,
-          symbol->name())};
-      if (subp->isFunction()) {
-        const auto &result{subp->result().name()};
-        msg->Attach(result, "Function result is '%s'"_en_US, result);
+    } else if (IsProcedure(*symbol)) {
+      if (auto *msg{context_.SayAt(x,
+              "Assignment to procedure '%s' is not allowed"_err_en_US,
+              symbol->name())}) {
+        if (auto *subp{symbol->detailsIf<semantics::SubprogramDetails>()}) {
+          if (subp->isFunction()) {
+            const auto &result{subp->result().name()};
+            msg->Attach(result, "Function result is '%s'"_en_US, result);
+          }
+        }
       }
     } else {
-      context_.SayAt(x, "Assignment to constant '%s' is not allowed"_err_en_US,
-          symbol->name());
+      context_.SayAt(
+          x, "Assignment to '%s' is not allowed"_err_en_US, symbol->name());
     }
   }
   fatalErrors_ = true;

flang/test/Semantics/assign04.f90

 end
 
 subroutine s8
-  !ERROR: Assignment to subprogram 's8' is not allowed
+  !ERROR: Assignment to procedure 's8' is not allowed
   s8 = 1.0
 end
 
 real function f9() result(r)
-  !ERROR: Assignment to subprogram 'f9' is not allowed
+  !ERROR: Assignment to procedure 'f9' is not allowed
   f9 = 1.0
 end
 
+subroutine s9
+  real f9a
+  !ERROR: Assignment to procedure 'f9a' is not allowed
+  f9a = 1.0
+  print *, f9a(1)
+end
+
 !ERROR: No explicit type declared for dummy argument 'n'
 subroutine s10(a, n)
   implicit none