This patch makes `==` and `<` for MemberDecorationInfo a friend function
and removes a `hasValue` field. `decorationValue` is also made an
`mlir::Attribute` so `UnitAttr` can be used to represent no-value. This
is consistent with how OpDecorate is handled in the deserializer. Using
`Attribute` will also enable handling non-integer values, however, there
seem to be no such decorations for struct members now.
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