That is, if a struct's generic requirements can't be deserialized,
drop the struct. This is the same logic that's already in play for
enums and (as of the previous commit) classes, so it should be pretty
well tested by now. (Hence the sole test I'm adding here, snuck into
superclass.swift because it's a superclass /constraint/ being tested.)
I don't know of any outstanding issues caused by this, but it was
weird to have it for enums and classes but not structs, so here we
are.
...instead of crashing. Also drop the class if its generic
requirements depend on a type that can't be loaded (instead of
crashing).
rdar://problem/50125674