Avoid a reverse map that is only used in diagnostics paths
r? `@petrochenkov`
iterating a map until a value matches and returning the key is bad obviously, but it happens very rarely and only on diagnostics paths. It would also be a lot cheaper with https://github.com/rust-lang/rust/pull/138995. Which is actually why I'm trying this out, that PR adds a new entry in `create_def`, which makes `create_def` show up in cachegrind. So I'm trying out if removing adding an entry in `create_def` is a perf improvement
bc05aaeeaa