Conditionally enable behaviors by a frontend flag.

Since the feature is incomplete and yet to be accepted or implemented as proposed, hide it behind an -enable-experimental-property-behaviors frontend flag.

include/swift/Basic/LangOptions.h

@@ -137,6 +137,9 @@ namespace swift {
     /// \brief Enable experimental "switch" pattern-matching features.
     bool EnableExperimentalPatterns = false;
 
+    /// \brief Enable experimental property behavior feature.
+    bool EnableExperimentalPropertyBehaviors = false;
+
     /// Should we check the target OSs of serialized modules to see that they're
     /// new enough?
     bool EnableTargetOSChecking = true;