[ty] Only calculate information for unresolved-reference subdiagnostic if we know we'll emit the diagnostic

[AlexWaygood]

Summary

This optimizes some of the logic added in #18444. In general, we only calculate information for subdiagnostics if we know we'll actually emit the diagnostic. The check to see whether we'll emit the diagnostic is work we'll definitely have to do whereas the the work to gather information for a subdiagnostic isn't work we necessarily have to do if the diagnostic isn't going to be emitted at all.

This PR makes us lazier about gathering the information we need for the subdiagnostic, and moves all the subdiagnostic logic into one function rather than having some unresolved-reference subdiagnostic logic in infer.rs and some in diagnostic.rs.

Test Plan

cargo test -p ty_python_semantic

[github-actions]

mypy_primer results

No ecosystem changes detected ✅

[carljm]

I don't see where this is delaying work compared to the prior version from #18444? It seems like in both the old and new version, all the work is in deciding whether an attribute exists in the first place, and in both old and new, that work is done if we are definitely going to emit a diagnostic, and not otherwise. Edit: ah, never mind, I forgot about the early return if context.report_lint(...) returns None.

But this looks like a nice refactor to collect more of the work in one place, either way!