Deprecate `-Winaccessible-code`
In !5100 (comment 333682), the special role of -Winaccessible-code came up. In particular, how it's not integrated well with the rest of how the pattern-match checker works today. For one, it's implemented somewhere in the typechecker, I think. But I also think its purpose is subsumed by -Woverlapping-patterns (which will warn for any inaccessible RHS, not just those caused by contradictory type constraints) noawadays.
I propose to deprecate the flag.