Limit 'address of ref' errors to safe code

[dkorpel]

Returning the address of a ref variable is rejected even in @system code, even though it is allowed as soon as there is an intermediate assignment: #13993 (comment)

This inconsistency makes that adding return scope, which should strictly improve safety compared to a parameter without any scope annotations, can break @system code. This blocks #14100

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[maxhaton]

LGTM