handle all type variants correctly in dropck

This fixes a few soundness bugs in dropck, so to anyone who relied on them,
this is a
[breaking-change]

Fixes #24086.
Fixes #25389.
Fixes #25598.
Fixes #25750.
Fixes #26641.
Fixes #26657.
Fixes #27240.
Fixes #27241.

Author: Ariel Ben-Yehuda