Auto merge of #37717 - nikomatsakis:region-obligations-pre, r=eddyb

Auto merge of #37717 - nikomatsakis:region-obligations-pre, r=eddyb

Refactoring towards region obligation

Two refactorings towards the intermediate goal of propagating region obligations through the `InferOk` structure (which in turn leads to the possibility of lazy normalization).

1. Remove `TypeOrigin` and add `ObligationCause`
    - as we converge subtyping and obligations and so forth, the ability to keep these types distinct gets harder
2. Propagate obligations from `InferOk` into the surrounding fulfillment context

After these land, I have a separate branch (which still needs a bit of work) that can make the actual change to stop directly adding subregion edges and instead propagate obligations. (This should also make it easier to fix the unsoundness in specialization around lifetimes.)

r? @eddyb