Refactor away inferred_obligations from the trait selector

[aravind-pg]

A small refactoring that removes the inferred_obligations field from SelectionContext in the trait selector in favor of threading nested obligations around explicitly. Closes #48672.

r? @nikomatsakis

[rust-highfive]

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[nikomatsakis]

@bors r+

[bors]

📌 Commit 81f0b96 has been approved by nikomatsakis

[nikomatsakis]

Just for reference, @aravind-pg and I had a short conversation on gitter where I pointed out that this may change behavior slightly, but it seems like the existing behavior was a bug and I think in practice it wouldn't have any effect anyway.