Explorer: Include self pattern in method type (#2935)

This fixes a bug where Explorer would not detect when:

- an implementation used a method to implement a class function in an interface
- an implementation used a class function to implement a method in an interface
- an implementation method used `addr self` when the interface method did not
- an interface method used `addr self` when the implementation method did not

at type checking time. This would then cause a crash at runtime.

Closes #2857

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

explorer/testdata/interface/fail_impl_missing_self.carbon

@@ -0,0 +1,24 @@
+// 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
+
+package ExplorerTest api;
+
+interface I {
+  fn G[self: Self]();
+}
+
+class C {
+  impl as I {
+    // CHECK:STDERR: COMPILATION ERROR: fail_impl_missing_self.carbon:[[@LINE+3]]: type error in member of implementation
+    // CHECK:STDERR: expected: fn [self: class C]() -> ()
+    // CHECK:STDERR: actual: fn () -> ()
+    fn G() { }
+  }
+}
+
+fn Main() -> i32 {
+  return 0;
+}