borrowck: Introduce BlameConstraint::to_obligation_cause_from_path()

[Enselic]

Before this commit, BlameConstraint is a bit hard to understand at first, because of its ObligationCause field. In practice, BlameConstraint is just a slimmed down version of an OutlivesConstraint from Vec<OutlivesConstraint>.

The artificial ObligationCause is only needed in one place. Rather than having that exception polute the lower level analysis, bubble it up as much as we can. That way, it becomes clearer what BlameConstraint actually is.

For reference, ObligationCause was introdued by #89249, but the codebase looked quite different back then.

This takes us one step closer (this is commit 1 of 4) towards the end result illustrated in #158623.

Notes

I have removed the comment

        // We try to avoid reporting a `ConstraintCategory::Predicate` as our best constraint.
        // Instead, we use it to produce an improved `ObligationCauseCode`.

because I think this existing comment is sufficient:

                // We handle predicates and opaque types specially; don't prioritize them here.
                ConstraintCategory::Predicate(_) | ConstraintCategory::OpaqueType => 4,

[rustbot]

r? @JohnTitor

rustbot has assigned @JohnTitor.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: borrowck, compiler
• borrowck, compiler expanded to 73 candidates
• Random selection from 16 candidates