privacy: no nominal visibility for assoc fns
Fixes#113860.
When `staged_api` is enabled, effective visibilities are computed earlier and this can trigger an ICE in some cases.
In particular, if a impl of a trait method has a visibility then an error will be reported for that, but when privacy invariants are being checked, the effective visibility will still be greater than the nominal visbility and that will trigger a `span_bug!`.
However, this invariant - that effective visibilites are limited to nominal visibility - doesn't make sense for associated functions.
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