Auto merge of #145377 - ChayimFriedman2:solver-def-id, r=lcnr

Switch next solver to use a specific associated type for trait def id

The compiler just puts `DefId` in there, but rust-analyzer uses different types for each kind of item.

See [the Zulip discussion](https://rust-lang.zulipchat.com/#narrow/channel/185405-t-compiler.2Frust-analyzer/topic/Implmentating.20New.20Trait.20Solver/near/534329794). In short, it will be a tremendous help to r-a to use specific associated types, while for the solver and the compiler it's a small change. So I ported `TraitId`, as a proof of concept and it's also likely the most impactful.

r? types

Author: bors