internal: Record import origins in ItemScope and PerNS
This records the import items definitions come from in the module scope (as well as what an import resolves to in an ItemScope). It does ignore glob imports as thats a lot more work for little to no gain, glob imports act as if the importing items are "inlined" into the scope which suffices for almost all use cases I believe (to my knowledge, attributes on them have little effect).
There is still a lot of work needed to make this available to the IDE layer, but this lays out the ground work for havin IDE layer support.
cc https://github.com/rust-lang/rust-analyzer/issues/14079
Properly infer types with type casts
This PR reenables `Expectation::Castable` (previous attempt at #14104, reverted by #14120) and implements type cast checks, which enable us to infer a bit more.
Castable expectations are relatively weak -- they only influence the inference if we cannot infer the types by other means. Therefore, we need to defer possible type unification with the casted type until we type check all expressions of the body. This PR adds a struct and slots in `InferenceContext` for the deferred cast checks (c.f. [`CastCheck`] in `rustc_hir_typeck`).
I only implemented the bits that affect the inference result. It should be possible to return type adjustments for well-formed casts and report diagnostics for invalid casts, but I'm leaving them for future work for now.
Fixes#11571Fixes#15246
[`CastCheck`]: da1d099f91/compiler/rustc_hir_typeck/src/cast.rs (L55)
- use `DefWithBodyId::as_generic_def_id()`
- add comments on `InferenceResult` invariant
- move local helper function to bottom to comply with style guide
