Auto merge of #121462 - compiler-errors:eq-and-sub, r=lcnr

Combine `Sub` and `Equate`

Combine `Sub` and `Equate` into a new relation called `TypeRelating` (that name sounds familiar...)

Tracks the difference between `Sub` and `Equate` via `ambient_variance: ty::Variance` much like the `NllTypeRelating` relation, but implemented slightly jankier because it's a more general purpose relation.

r? lcnr

Author: bors