Don't ICE in has_self_borrows when coroutine captures-by-ref ty is still inferred

[swjng]

Closes #155999.

coroutine_captures_by_ref_ty is a fresh next_ty_var until upvar
inference unifies it to an FnPtr. If has_self_borrows is reached
before that — e.g. when the body contains an unresolved as _ cast —
the _ => panic!() arm fires:

//@ edition:2021
fn needs_fn_mut<T>(x: impl FnMut() -> T) {
    needs_fn_mut(async || x as _)
}

Treat the Infer case like Error: report self-borrows defensively
so the closure falls back to FnOnce-only and the caller surfaces a
normal type-inference error instead of an ICE.

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