[mlir][IR] Generalize `DenseElementsAttr` to custom element types (#179122)

`DenseElementsAttr` supports only a hard-coded list of element types:
`int`, `index`, `float`, `complex`. This commit generalizes the
`DenseElementsAttr` infrastructure: it now supports arbitrary element
types, as long as they implement the new `DenseElementTypeInterface`.

The `DenseElementTypeInterface` has the following helper functions:
- `getDenseElementBitSize`: Query the size of an element in bits. (When
storing an element in memory, each element is padded to a full byte.
This is an existing limitation of the `DenseElementsAttr`; with an
exception for `i1`.)
- `convertToAttribute`: Attribute factory / deserializer. Converts bytes
into an MLIR attribute. The attribute provides the assembly format /
printer for a single element.
- `convertFromAttribute`: Serializer. Converts an MLIR attribute into
bytes.

Note: `convertToAttribute` / `convertFromAttribute` are mainly for
writing test cases. For performance reasons, `DenseElementsAttr` users
should work with raw bytes / elements and avoid any API that
materializes MLIR attributes. However, MLIR attributes typically have
human-readable parsers/printers, making them suitable for lit tests and
debugging.

This PR introduces an additional assembly format for
`DenseElementsAttrs`. There are now two formats. (The existing one is
kept for compatibility reasons.)
- Literal-first (existing): `dense<[1, 2, 3]> : tensor<3xi32>`
- Type-first (new): `dense<tensor<3xi32> : [1 : i32, 2 : i32, 3 : i32]>`

The new syntax is needed to disambiguate between "literal" (e.g., `1`)
and attribute (e.g., `1 : i32`) when parsing the first token. In the
literal-first syntax, we only parse literals. In the type-first syntax,
we only parse attributes.

The existing `int`, `index`, `float`, `complex` types also implement the
`DenseElementTypeInterface`. This allows us to implement
`DenseElementsAttr::get` and `AttributeElementIterator::operator*` in a
generic way.

RFC:
https://discourse.llvm.org/t/rfc-allow-custom-element-types-in-denseelementattr/89656