find_matching_impl() called resolve_vars_if_possible(), but we rely on
unresolved region inference variables not escaping. Ensure that we
substitute these variables.
Without this fix, we can panic when trying to resolve the variable in
the wrong inference context. See the new unit test.
This seems to have always been wrong, but the panic only started
occurring due when libcore changed in
rust-lang/rust#136006.
AI disclosure: Code partly written by GPT-5 Codex. I've bisected and
confirmed the repro, and reviewed all the code, but possibly there's a
nicer place to fix this logic.