Add 6.8 output for FD3

testsuite/tests/ghc-regress/typecheck/should_compile/FD3.stderr-6.8

+
+FD3.hs:15:14:
+    Could not deduce (MkA (String, a) a) from the context ()
+      arising from a use of `mkA' at FD3.hs:15:14-18
+    Possible fix:
+      add (MkA (String, a) a) to the context of
+        the type signature for `translate'
+      or add an instance declaration for (MkA (String, a) a)
+    In the expression: mkA a
+    In the definition of `translate': translate a = mkA a