Move orphan instance/rule warnings to typechecker/desugarer.
Summary: Instead of doing these warnings at MkIface time, we do them when we create the instances/rules in the typechecker/desugarer. Emitting warnings for auto-generated instances was a pain (since the specialization monad doesn't have the capacity to emit warnings) so instead I just deprecated -fwarn-auto-orphans. Auto rule orphans are pretty harmless anyway: they don't cause interface files to be eagerly loaded in. Signed-off-by: Edward Z. Yang <firstname.lastname@example.org> Test Plan: validate Reviewers: simonpj, austin, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1297