Rollup merge of #143414 - dianne:box-usefulness-cleanup, r=Nadrieril

remove special-casing of boxes from match exhaustiveness/usefulness analysis

As a first step in replacing `box_patterns` with `deref_patterns`, this treats box patterns as deref patterns in the THIR and exhaustiveness analysis. This allows a bunch of special-casing to be removed. The emitted MIR is unchanged.

Incidentally, this fixes a bug caused by box patterns being treated like structs rather than pointers, where enabling `exhaustive_patterns` (rust-lang/rust#51085) could give rise to spurious `unreachable_patterns` lints on arms required for exhaustiveness. Following the lint's advice to remove the match arm would result in an error. I'm not sure what the current state of `exhaustive_patterns` is with regard to reference/box opsem, or whether there's any intention to have `unreachable_patterns` be more granular than the whole arm, but regardless this should hopefully make them easier to handle consistently.

Tracking issue for deref patterns: rust-lang/rust#87121

r? `@Nadrieril`

Author: matthiaskrgr