[PrintAsObjC] Use of imported generics require the full definition. (#5518)
I'm not sure why this didn't occur to me in 8282160d: of course if you
see a generic type with arguments, you need to see the @interface for
that type in order to supply the arguments. Maybe I was thinking the
generated interface would automatically import anything the module
itself imports, but that hasn't ever been true.
rdar://problem/28738008