Rollup merge of #99000 - JulianKnodt:allow_resolve_no_substs, r=lcnr

Rollup merge of #99000 - JulianKnodt:allow_resolve_no_substs, r=lcnr

Move abstract const to middle

Moves AbstractConst (and all associated methods) to rustc middle for use in `rustc_infer`.
This allows for const resolution in infer to use abstract consts to walk consts and check if
they are resolvable.

This attempts to resolve the issue where `Foo<{ concrete const }, generic T>` is incorrectly marked as conflicting, and is independent from the other issue where nested abstract consts must be resolved.

r? `@lcnr`