hygiene: Ensure uniqueness of `SyntaxContextData`s `SyntaxContextData`s are basically interned with `SyntaxContext`s working as indices, so they are supposed to be unique. However, currently duplicate `SyntaxContextData`s can be created during decoding from metadata or incremental cache. This PR fixes that. cc https://github.com/rust-lang/rust/pull/129827#discussion_r1759074553