[flang] Allow a construct entity as a concurrent-header index

[klausler]

Summary

A construct entity of an ASSOCIATE or SELECT TYPE construct
should be acceptable as an index variable of a concurrent-header in
a FORALL or DO CONCURRENT, so long as it also satisfies other
requirements.

[jeanPerier]

LGTM

flang/lib/Semantics/resolve-names.cpp

if (prev) {
     name.symbol = nullptr;
   }
   auto &symbol{DeclareObjectEntity(name)};
   if (symbol.GetType()) {
     // type came from explicit type-spec
   } else if (!prev) {
     ApplyImplicitRules(symbol);
   } else {
-    const Symbol &prevRoot{ResolveAssociations(*prev)};
+    const Symbol &prevRoot{prev->GetUltimate()};
     // prev could be host- use- or construct-associated with another symbol
     if (!prevRoot.has<ObjectEntityDetails>() &&
-        !prevRoot.has<EntityDetails>()) {
+        !prevRoot.has<AssocEntityDetails>()) {
       Say2(name, "Index name '%s' conflicts with existing identifier"_err_en_US,
           *prev, "Previous declaration of '%s'"_en_US);
       context().SetError(symbol);
       return;
     } else {
       if (const auto *type{prevRoot.GetType()}) {
         symbol.SetType(*type);
       }

flang/test/Semantics/forall01.f90

subroutine forall6
   type(t) :: a(10)
   real, target :: b(10)
   forall(i=1:10)
     a(i)%p => b(i)
     !WARNING: FORALL index variable 'i' not used on left-hand side of assignment
     a(1)%p => b(i)
   end forall
 end
+
+subroutine forall7(x)
+  integer :: iarr(1)
+  real :: a(10)
+  class(*) :: x
+  associate (j => iarr(1))
+    forall (j=1:size(a))
+      a(j) = a(j) + 1
+    end forall
+  end associate
+  associate (j => iarr(1) + 1)
+    forall (j=1:size(a))
+      a(j) = a(j) + 1
+    end forall
+  end associate
+  select type (j => x)
+  type is (integer)
+    forall (j=1:size(a))
+      a(j) = a(j) + 1
+    end forall
+  end select
+end subroutine