Fix wrong frame offsets for locals of alternate event/hook prototypes

Local frame offsets were being assigned based on number of the alternate
prototype's parameters, which may end up having less total parameters
than the canonical prototype, causing the local value to incorrectly
overwrite an event/hook argument value.

src/Type.h

@@ -429,6 +429,8 @@ public:
 		bool deprecated;
 		std::string deprecation_msg;
 		RecordTypePtr args;
+		// Maps from parameter index in canonical prototype to
+		// parameter index in this alternate prorotype.
 		std::map<int, int> offsets;
 	};