Fix a simple bug.

Apparently I had not tested this with anything that actually
set a flag to 'false'.

Author: rjmccall

include/swift/Basic/FlagSet.h

@@ -60,7 +60,7 @@ class FlagSet {
     if (value) {
       Bits |= maskFor<Bit>();
     } else {
-      Bits |= ~maskFor<Bit>();
+      Bits &= ~maskFor<Bit>();
     }
   }