Custom attributes were being skipped as their attribute name started
with a "_". Underscored attributes are typically unstable and thus
shouldn't be printed, but "_" is not being used for that purpose in the
case of "_custom".
Rather than checking for "_" or "__", just use `UserInaccessible`. This
property is used for attributes that shouldn't be shown to users in eg.
completion/printing/etc.
Resolves rdar://99029554.
