Move our paths over to diagnostic items

[Manishearth]

Currently we have a ton of hardcoded paths for stdlib types.

Rustc has a way of tagging types for diagnostics via a "diagnostic item" API (rust-lang/rust#60966). We should go through rustc and tag most of these diagnostic items, and replace the hardcoded paths with hardcoded diagnostic item names instead. We can still keep the path framework around for rapid prototyping, but have a comment recommending people switch to diagnostic items whenever possible.

[flip1995]

Marking it as tracking issue and pinning it, since this is probably done in multiple steps.