Reclassify test

I believe #4674 had the wrong resolution. The test leads to spurious warnings for compiler-generated pattern matches.
I believe it's better to revert the decision and check only explicit isInstanceOf, never patterns.

Note the scalac does not check patterns either.

Author: odersky

tests/neg-custom-args/fatal-warnings/i4674.scala renamed to tests/pos-special/fatal-warnings/i4674.scala

@@ -2,7 +2,7 @@ class Test {
   def test(x: String) = {
     x.foreach {
       case 's' => println("s")
-      case c: Char => println(c) // error: type test always succeeds
+      case c: Char => println(c) // should compile without warning
     }
   }
 }