Phab:D493 accidentally changed the way GHC generics looks up `Fixity`
information when deriving `Generic` or `Generic1`. Before, a `Fixity` of
`Infix` would be given only if a data constructor was declared infix,
but now, `Infix` is given to any data constructor that has a fixity
declaration (not to be confused with being declared infix!). This commit
reverts back to the original behavior for consistency's sake.

Fixes #11358.

Test Plan: ./validate

Reviewers: kosmikus, dreixel, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1740

GHC Trac Issues: #11358