[mlir] Refactor ElementsAttr into an AttrInterface

This revision refactors ElementsAttr into an Attribute Interface.
This enables a common interface with which to interact with
element attributes, without needing to modify the builtin
dialect. It also removes a majority (if not all?) of the need for
the current OpaqueElementsAttr, which was originally intended as
a way to opaquely represent data that was not representable by
the other builtin constructs.

The new ElementsAttr interface not only allows for users to
natively represent their data in the way that best suits them,
it also allows for efficient opaque access and iteration of the
underlying data. Attributes using the ElementsAttr interface
can directly expose support for interacting with the held
elements using any C++ data type they claim to support. For
example, DenseIntOrFpElementsAttr supports iteration using
various native C++ integer/float data types, as well as
APInt/APFloat, and more. ElementsAttr instances that refer to
DenseIntOrFpElementsAttr can use all of these data types for
iteration:

```c++
DenseIntOrFpElementsAttr intElementsAttr = ...;

ElementsAttr attr = intElementsAttr;
for (uint64_t value : attr.getValues<uint64_t>())
  ...;
for (APInt value : attr.getValues<APInt>())
  ...;
for (IntegerAttr value : attr.getValues<IntegerAttr>())
  ...;
```

ElementsAttr also supports failable range/iterator access,
allowing for selective code paths depending on data type
support:

```c++
ElementsAttr attr = ...;
if (auto range = attr.tryGetValues<uint64_t>()) {
  for (uint64_t value : *range)
    ...;
}
```

Differential Revision: https://reviews.llvm.org/D109190