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d74c6b11 - [mlir][IR] Generalize `DenseElementsAttr` to custom element types (#183920)

[mlir][IR] Generalize `DenseElementsAttr` to custom element types (#183920) `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 This is a re-upload of #179122.