Type families: test for tau typeness of rhs of type instances

testsuite/tests/ghc-regress/indexed-types/should_fail/Simple12.hs

+{-# OPTIONS -ftype-families -fglasgow-exts #-}
+
+module ShouldFail where
+
+type family C a :: *
+type instance C Int = forall a. [a]
+

testsuite/tests/ghc-regress/indexed-types/should_fail/Simple12.stderr

+
+Simple12.hs:6:0:
+    Illegal polymorphic type in type instance: forall a. [a]

testsuite/tests/ghc-regress/indexed-types/should_fail/all.T

@@ -18,6 +18,7 @@ test('Simple11a', normal, compile_fail, [''])
 test('Simple11b', normal, compile_fail, [''])
 test('Simple11c', normal, compile_fail, [''])
 test('Simple11d', normal, compile_fail, [''])
-test('Over', normal, multimod_compile_fail, ['OverD', '-no-hs-main -no-link'])
+test('Simple12', normal, compile_fail, [''])
 
+test('Over', normal, multimod_compile_fail, ['OverD', '-no-hs-main -no-link'])
 clean(['OverA.hi', 'OverA.o', 'OverB.hi', 'OverB.o', 'OverC.hi', 'OverC.o'])