Remove `Identifier` usages for isolating exception names (#5797)
## Summary
The motivating change here is to remove `let range =
except_handler.try_identifier().unwrap();` and instead just do
`name.range()`, since exception names now have ranges attached to them
by the parse. This also required some refactors (which are improvements)
to the built-in attribute shadowing rules, since at least one invocation
relied on passing in the exception handler and calling
`.try_identifier()`. Now that we have easy access to identifiers, we can
remove the whole `AnyShadowing` abstraction.
