[flang] Check that DO index variables are definable

[klausler]

Summary

We're letting immutable objects appear as DO index variables;
catch and diagnose this error.

flang/lib/Semantics/check-do-forall.cpp

 //===----------------------------------------------------------------------===//
 
 #include "check-do-forall.h"
+#include "definable.h"
 #include "flang/Common/template.h"
 #include "flang/Evaluate/call.h"
 #include "flang/Evaluate/expression.h"

       if (!IsVariableName(*symbol)) {
         context_.Say(
             sourceLocation, "DO control must be an INTEGER variable"_err_en_US);
+      } else if (auto why{WhyNotDefinable(sourceLocation,
+                     context_.FindScope(sourceLocation), DefinabilityFlags{},
+                     *symbol)}) {
+        context_
+            .Say(sourceLocation,
+                "'%s' may not be used as a DO variable"_err_en_US,
+                symbol->name())
+            .Attach(std::move(*why));
       } else {
         const DeclTypeSpec *symType{symbol->GetType()};
         if (!symType) {

flang/test/Semantics/definable03.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+subroutine sub(j)
+  integer, intent(in) :: j
+  !ERROR: 'j' may not be used as a DO variable
+  !BECAUSE: 'j' is an INTENT(IN) dummy argument
+  do j = 1, 10
+  end do
+end