Tracking when Object.toString is deterministic

[mernst]

Sometimes, all the values flowing to a call to toString have overridden Object.toString, even if the expression's declared type doesn't do so.
The Determinism Checker should track this information.
This will enable it to determine what calls to Object.toString are legal, eliminating a source of false positives.

One possible design would have this type hierarchy:

@OverridesToStringUnknown  -- the value's run-time class might not override Object.toString
       |
@OverridesToString  -- the value's run-time class definitely overrides Object.toString