[mlir][DenseElementsAttr] ::get for Tensor and Complex

[rriddle]

This patch doesn't look correct. This is just going to lead to an assert/crash in the other code path, the else branch there doesn't have any support for tensor or complex.

[atondwal]

I expected this to Just Work, since it seems like it did before 0d5caa8, and because RankedTensorType and ComplexType just have arrays as storage types, same as DenseElementsAttr, but I'm not familiar with the invariant relevant to this bit of code.
AFAICT there aren't any failing assertions in the else branch; was able to use this to successfully construct a DEA of complex<f32>.
(hadn't meant to send this out for review just yet without checking in the tests; I think I misunderstood the cl2git flags)

mlir/lib/IR/BuiltinAttributes.cpp

 bool DenseElementsAttr::classof(Attribute attr) {
   return attr.isa<DenseIntOrFPElementsAttr, DenseStringElementsAttr>();
 }
 
 DenseElementsAttr DenseElementsAttr::get(ShapedType type,
                                          ArrayRef<Attribute> values) {
   assert(hasSameElementsOrSplat(type, values));
 
-  // If the element type is not based on int/float/index, assume it is a string
-  // type.
+  // If the element type is an int/float/index or a complex/tensor assume it is
+  // a string type.
   auto eltType = type.getElementType();
-  if (!type.getElementType().isIntOrIndexOrFloat()) {
+  if (!eltType.isIntOrIndexOrFloat() && !eltType.isa<TensorType>() &&
+      !eltType.isa<ComplexType>()) {
     SmallVector<StringRef, 8> stringValues;
     stringValues.reserve(values.size());
     for (Attribute attr : values) {
       assert(attr.isa<StringAttr>() &&
              "expected string value for non integer/index/float element");
       stringValues.push_back(attr.cast<StringAttr>().getValue());
     }
     return get(type, stringValues);