Make addr me: Self* work for interface methods. (#2374)

Fixes #2223.

explorer/testdata/interface/addr_me.carbon

@@ -0,0 +1,50 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{explorer-run}
+// RUN: %{explorer-run-trace}
+// CHECK:STDOUT: 75
+// CHECK:STDOUT: 100
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+interface Vector {
+  fn Zero() -> Self;
+  fn Add[addr me: Self*](b: Self);
+  fn Scale[addr me: Self*](v: i32);
+}
+
+class Point {
+  var x: i32;
+  var y: i32;
+  impl as Vector {
+    fn Zero() -> Self {
+      return {.x = 1, .y = 1};
+    }
+    fn Add[addr me: Self*](b: Self) {
+      (*me).x = (*me).x + b.x;
+      (*me).y = (*me).y + b.y;
+    }
+    fn Scale[addr me: Self*](v: i32) {
+      (*me).x = (*me).x * v;
+      (*me).y = (*me).y * v;
+    }
+  }
+}
+
+fn AddAndScaleGeneric[T:! Vector](p: T*, s: i32) {
+  (*p).Add(T.Zero());
+  (*p).(Vector.Scale)(s);
+  (*p).(T.(Vector.Scale))(s);
+}
+
+fn Main() -> i32 {
+  var a: Point = {.x = 2, .y = 3};
+  AddAndScaleGeneric(&a, 5);
+  Print("{0}", a.x);
+  Print("{0}", a.y);
+  return 0;
+}