Rollup merge of #125664 - compiler-errors:trace-tweaks, r=lcnr

Tweak relations to no longer rely on `TypeTrace`

Remove `At::trace`, and inline all of the `Trace::equate`,etc methods into `At`.

The only nontrivial change is that we use `AliasTerm` to relate two unevaluated consts in the old-solver impl of `ConstEquate`, since `AliasTerm` does implement `ToTrace` and will relate the args structurally (shallowly).

r? lcnr

Author: jieyouxu