Use FlexibleInstances for `Outputable (* p)` instead of match-all instances with equality constraints

In #17304, Richard and Simon dicovered that using `-XFlexibleInstances`
for `Outputable` instances of AST data types means users can provide orphan
`Outputable` instances for passes other than `GhcPass`.

Type inference doesn't currently to suffer, and Richard gave an example
in #17304 that shows how rare a case would be where the slightly worse
type inference would matter.

So I went ahead with the refactoring, attempting to fix #17304.

1 files changed, 0 insertions, 0 deletions

diff --git a/utils/haddock b/utils/haddock
-Subproject f0b5a2043ff6c527e55fab228d37ee698ce8726
+Subproject fad111e9d3de1a2e86837d3e6f72fe0cf2f6c0a