Rollup merge of #135426 - compiler-errors:no-resolve-assoc-ty, r=lcnr

Assert that `Instance::try_resolve` is only used on body-like things

`Instance::resolve` is not set up to resolve items that are not body-like things. The logic in `resolve_associated_item` very much encodes this assumption:

https://github.com/rust-lang/rust/blob/e7ad3ae331bf2716389c10e01612e201a7f98c8d/compiler/rustc_ty_utils/src/instance.rs#L96-L386

However, some diagnostics were using `Instance::resolve` on an associated type, and it was simply a lucky coicidence that nothing went wrong.

This PR adds an assertion to make sure we won't do this again in the future, and fixes two callsites:
1. `call_kind` which returns a `CallKind` enum to categorize what a call in MIR comes from, and was using `Instance::resolve` to point at the associated type `Deref::Target` for a specific self ty.
2. `MirBorrowckCtxt::explain_deref_coercion`, which was doing the same thing.

The logic was replaced with `specialization_graph::assoc_def`, which is the proper way of fetching the right `AssocItem` for a given impl.

r? `@lcnr` or re-roll :)

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