Mention DerivingStrategies in the warning when DAC and GND are both enabled

Summary:

When DeriveAnyClass and GeneralizedNewtypeDeriving are both enabled,
GHC prints out a warning that specifies the strategy it used to
derive a class. This patch updates the warning to mention that users
may pick a particular strategy by using DerivingStrategies.

Test plan: make test TEST=T16179

compiler/typecheck/TcDeriv.hs

@@ -1640,7 +1640,10 @@ mkNewTypeEqn
                      [ text "Both DeriveAnyClass and"
                        <+> text "GeneralizedNewtypeDeriving are enabled"
                      , text "Defaulting to the DeriveAnyClass strategy"
-                       <+> text "for instantiating" <+> ppr cls ]
+                       <+> text "for instantiating" <+> ppr cls
+                     , text "Use DerivingStrategies to pick"
+                       <+> text "a different strategy"
+                      ]
                  mk_originative_eqn DerivSpecAnyClass
                -- CanDeriveStock
                CanDeriveStock gen_fn -> mk_originative_eqn $

testsuite/tests/deriving/should_compile/T16179.hs

+{-# LANGUAGE DeriveAnyClass             #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+
+module T16179 where
+
+class C a
+newtype T a = MkT a deriving C

testsuite/tests/deriving/should_compile/T16179.stderr

+T16179.hs:7:30: warning:
+     Both DeriveAnyClass and GeneralizedNewtypeDeriving are enabled
+      Defaulting to the DeriveAnyClass strategy for instantiating C
+      Use DerivingStrategies to pick a different strategy
+     In the newtype declaration for ‘T’

testsuite/tests/deriving/should_compile/all.T

@@ -114,3 +114,4 @@ test('T15290c', normal, compile, [''])
 test('T15290d', normal, compile, [''])
 test('T15398', normal, compile, [''])
 test('T15637', normal, compile, [''])
+test('T16179', normal, compile, [''])