use equality in the coerce-unsized check
This seems both to be a safe, conservative choice, and it sidesteps the cycle in #41849. Note that, before I converted variance into proper queries, we were using a hybrid of subtyping and equality, due to the presence of a flag that forced invariance if variance had not yet been computed. (Also, Coerce Unsized is unstable.)
Fixes #41936.
r? @eddyb