treat box patterns as deref patterns in THIR and usefulness analysis

This removes special-casing of boxes from `rustc_pattern_analysis`, as a
first step in replacing `box_patterns` with `deref_patterns`.
Incidentally, it fixes a bug caused by box patterns being represented as
structs rather than pointers, where `exhaustive_patterns` could generate
spurious `unreachable_patterns` lints on arms required for
exhaustiveness; following the lint's advice would result in an error.

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