Merge #6189

6189: adt: correctly inherit field visibility from enum r=jonas-schievink a=jonas-schievink

Previously, "find all references" on a variant field wouldn't find any
references outside the defining module. This is because variant fields
were incorrectly assumed to be private, like struct fields without
explicit visibility, but they actually inherit the enum's visibility.

bors r+ :robot:

Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>