This is the second (and hopefully last) fix needed to make TH handle
GADTs properly (after D1465). This Diff addresses some issues with infix
GADT constructors, specifically:

* Before, you could not determine if a GADT constructor was declared
  infix because TH did not give you the ability to determine if there is
  a //user-specified// fixity declaration for that constructor. The
  return type of `reifyFixity` was changed to `Maybe Fixity` so that it
  yields `Just` the fixity is there is a fixity declaration, and
  `Nothing` otherwise (indicating it has `defaultFixity`).
* `DsMeta`/`Convert` were changed so that infix GADT constructors are
  turned into `GadtC`, not `InfixC` (which should be reserved for
  Haskell98 datatype declarations).
* Some minor fixes to the TH pretty-printer so that infix GADT
  constructors will be parenthesized in GADT signatures.

Fixes #11345.

Test Plan: ./validate

Reviewers: goldfire, austin, bgamari, jstolarek

Reviewed By: jstolarek

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1744

GHC Trac Issues: #11345