Template Haskell allows reification of fixity for infix functions and
data constructors, and not for infix types. This adds a `Fixity` field
to the relevant `Info` constructors that can have infix types (`ClassI`,
`TyConI`, and `FamilyI`).

I don't think that `VarI` or `PrimTyConI` can be infix, but I could be
wrong.

Test Plan: ./validate

Reviewers: austin, goldfire, bgamari

Reviewed By: goldfire, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1109

GHC Trac Issues: #10704