Add support for Unit Attributes.

    A unit attribute is an attribute that represents a value of `unit` type. The
    `unit` type allows only one value forming a singleton set. This attribute value
    is used to represent attributes that only have meaning from their existence.

    One example of such an attribute could be the `swift.self` attribute. This attribute indicates that a function parameter is the self/context
    parameter. It could be represented as a boolean attribute(true or false), but a
    value of false doesn't really bring any value. The parameter either is the
    self/context or it isn't.

    ```mlir {.mlir}
    // A unit attribute defined with the `unit` value specifier.
    func @verbose_form(i1 {unitAttr : unit})

    // A unit attribute can also be defined without the `unit` value specifier.
    func @simple_form(i1 {unitAttr})
    ```

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PiperOrigin-RevId: 245254045