[MLIR] Add explicit using to disambiguate between multiple implementations from base classes (NFC)

Both of DenseElementsAttr and ElementsAttrTrait define the method of
getElementType, this commit makes it available on
DenseIntOrFPElementsAttr and DenseStringElementsAttr.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D116389

Author: lipracer

mlir/include/mlir/IR/BuiltinAttributes.td

@@ -175,6 +175,7 @@ def Builtin_DenseIntOrFPElementsAttr : Builtin_Attr<
   let extraClassDeclaration = [{
     using DenseElementsAttr::empty;
     using DenseElementsAttr::getNumElements;
+    using DenseElementsAttr::getElementType;
     using DenseElementsAttr::getValues;
     using DenseElementsAttr::isSplat;
     using DenseElementsAttr::size;
@@ -312,6 +313,7 @@ def Builtin_DenseStringElementsAttr : Builtin_Attr<
   let extraClassDeclaration = [{
     using DenseElementsAttr::empty;
     using DenseElementsAttr::getNumElements;
+    using DenseElementsAttr::getElementType;
     using DenseElementsAttr::getValues;
     using DenseElementsAttr::isSplat;
     using DenseElementsAttr::size;