Rollup merge of #112869 - compiler-errors:sketchy-new-select, r=lcnr

Implement selection via new trait solver

Implements selection via the new solver by calling into `assemble_and_evaluate_candidates`, doing a very light-weight "winnow" to choose one candidate over the others, and then re-confirming that candidate outside of the `EvalCtxt` in order to compute the information necessary for the `ImplSource`.

This selection routine is best effort -- if it receives an ambiguous response from the `EvalCtxt`, then it may return ambiguity if the work to re-confirm the goal is not "easy" -- right now, that means everything except for object and impl goals. Since we don't want to reimplement all of the work of the `compute_builtin_*` methods in the solver internals, we bail out if we encounter ambiguity more often than the old solver. This should only barely affect [method selection](https://github.com/rust-lang/rust/blob/f798ada7babac06d4611b0b3a6079d8399ab58ab/compiler/rustc_hir_typeck/src/method/probe.rs#L1447) and not codegen. It can be made more precise later if needed.

r? `@lcnr`

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