Auto merge of #49822 - matthewjasper:dropck-closures, r=nikomatsakis

Access individual fields of tuples, closures and generators on drop.

Fixes #48623, by extending the change in #47917 to closures. Also does this for tuples and generators for consistency.

Enums are unchanged because there is now way to borrow `*enum.field` without borrowing `enum.field` at the moment, so any error would be reported when the enum goes out of scope. Unions aren't changed because unions they don't automatically drop their fields.

r? @nikomatsakis