Make the Div and Mod type families `infixl 7`

Commit fa8035e3 added `Div`
and `Mod` type families to `GHC.TypeNats`. However, they did not add
the corresponding fixities! Currently, we have that both `div` and
`mod` (at the value level) are `infixl 7`, so we should adopt the
same fixities for the type-level `Div` and `Mod` as well.

Test Plan: It compiles

Reviewers: hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #14640

Differential Revision: https://phabricator.haskell.org/D4291

(cherry picked from commit 303106d5)

libraries/base/GHC/TypeNats.hs

@@ -99,7 +99,7 @@ instance Read SomeNat where
 
 infix  4 <=?, <=
 infixl 6 +, -
-infixl 7 *
+infixl 7 *, `Div`, `Mod`
 infixr 8 ^
 
 -- | Comparison of type-level naturals, as a constraint.