GH-927: Fix circumvention of evaluation order in 'when' conditions

Historically, a 'when' condition performed an AST-traversal to locate
any index-expressions like `x[9]` and evaluated them so that it could
register the associated value as something for which it needs to receive
"modification" notifications.

Evaluating arbitrary expressions during an AST-traversal like that ignores
the typical order-of-evaluation/short-circuiting you'd expect if the
condition was evaluated normally, from its root expression.

Now, a new subclass of IndexExpr is used to keep track of all IndexExpr
results in the context of evaluating a 'when' condition without having
to do a secondary AST-traversal-and-eval.  i.e. the first evaluation of
the full 'when' condition follows the typical expression-evaluation
semantics (as always), but additionally now captures all the values
a Trigger needs to monitor for modifications.

src/Trigger.h

@@ -91,7 +91,7 @@ private:
 	friend class TriggerTraversalCallback;
 	friend class TriggerTimer;
 
-	void Init();
+	void Init(std::vector<IntrusivePtr<Val>> index_expr_results);
 	void Register(ID* id);
 	void Register(Val* val);
 	void UnregisterAll();