[flang] Accept INDEX(..., BACK=array)

The intrinsic table entry for INDEX mistakenly required
the optional BACK= argument to be scalar, but it's an
elemental intrinsic that can accept a conforming array.

Differential Revision: https://reviews.llvm.org/D117700

flang/lib/Evaluate/intrinsics.cpp

@@ -486,7 +486,7 @@ static const IntrinsicInterface genericIntrinsicFunction[]{
     {"image_status", {{"image", SameInt}, OptionalTEAM}, DefaultInt},
     {"index",
         {{"string", SameChar}, {"substring", SameChar},
-            {"back", AnyLogical, Rank::scalar, Optionality::optional},
+            {"back", AnyLogical, Rank::elemental, Optionality::optional},
             DefaultingKIND},
         KINDInt},
     {"int", {{"a", AnyNumeric, Rank::elementalOrBOZ}, DefaultingKIND}, KINDInt},