Use AST nodes as generic arg deduction keys (#982)

executable_semantics/interpreter/interpreter.cpp

@@ -105,7 +105,7 @@ void Interpreter::InitEnv(const Declaration& d, Env* env) {
       for (Nonnull<const GenericBinding*> deduced :
            func_def.deduced_parameters()) {
         AllocationId a =
-            heap_.AllocateValue(arena_->New<VariableType>(deduced->name()));
+            heap_.AllocateValue(arena_->New<VariableType>(deduced));
         new_env.Set(deduced->name(), a);
       }
       Nonnull<const FunctionValue*> f = arena_->New<FunctionValue>(&func_def);