Currently, ABI classification attributes are hidden in an internal package, and users should use the pre-defined layout constants in CLinker to link up and down calls.

But, this makes it hard for tools such as jextract to use these layouts as dynamic constants, since the ABI attributes have to be filtered out manually, resulting in code duplication between jextract and ValueLayout's own describeConstable method.

Using a subtype of ValueLayout (e.g. CValueLayout) instead, allows us to embed this information more directly, probably removing the need for layout attributes altogether. It also allows us to override the describeConstable method, to generate a dynamic constant that references the constant field in CLinker instead.