Add "diagnostic items" which behave like lang items

[Manishearth]

It would be nice to have a (rustc-only) system of that lets us annotate structs with things like #[diagnostic_item="String"].

This lets us improve the diagnostics in special cases involving that type (e.g. #39116 (comment)). Clippy currently string-compares the path to do this, but that's hacky and I would rather we avoided that code in rustc.

It's something that would only get hit when we have an error case, so we probably can just store these in a hash map or something.

cc @jonathandturner @nikomatsakis

[Manishearth]

More details in #58070 (comment)

cc @oli-obk @phansch

[Manishearth]

Copying over the relevant comment:

We add a new attribute to the compiler: #[diagnostic = "debug_trait"] and similar so we can stop checking for lang items and instead just check for diagnostic-specific items.

This will be a bit more involved. Essentially we need to

1. duplicate the get_lang_items query. Though I don't think there's any point in having a method per diagnostic item.
2. So I suggest we just make the query return a FxHashMap<Symbol, NodeId>.
3. We can then build a wrapper method on the TyCtxt that maps a &str to an Option<NodeId> for user convenience
4. Throw the new attribute on a bunch of types (basically go through https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/utils/paths.rs and just flag all the things in that list)
5. see if there are any rustc diagnostics that could benefit from this
6. replace all uses of util/path.rs with a use of the &str -> Option<NodeId> method
7. complain to @oli-obk that they didn't think this through and there's uses of util/path.rs that do something different
8. figure out if we need any of this in EarlyLintPasses and make them LateLintPasses
9. see 7.