Various improvements to function overload resolution.

Generalize the overload resolution code to be based on estimating a
  cost for various overload options and picking the one with the
  minimal cost.
Add a step that considers type conversions that are guaranteed to
  not lose information in function overload resolution.
Print better diagnostics when we can't find an unambiguous match.

expr.h

@@ -269,7 +269,7 @@ public:
 
 private:
     void resolveFunctionOverloads(bool exactMatchOnly);
-    bool tryResolve(bool (*matchFunc)(Expr *, const Type *));
+    bool tryResolve(int (*matchFunc)(Expr *, const Type *));
 };