Get out of instance localization

Stream: general

[Yaël Dillies]

What's the right way to use localized instances in a non localized one? docs#set.set_semiring.non_unital_non_assoc_semiring isn't localized (and arguably shouldn't be), but it uses docs#set.has_mul which is localized in pointwise. Surely I shouldn't copy over the content of set.has_mul? I'm hitting this in #14145.