[mlir][spirv] Add support for structs decorations (#149793)
An alternative implementation could use `ArrayRef` of `NamedAttribute`s
or `NamedAttrList` to store structs decorations, as the deserializer
uses `NamedAttribute`s for decorations. However, using a custom struct
allows us to store the `spirv::Decoration`s directly rather than its
name in a `StringRef`/`StringAttr`.