Explorer: Support class extension (#1946)

Support class extension with `extends`
Relates to #1881 

Add support for:
* Class extension with `extends`
* Using base class methods and members

explorer/testdata/class/class_inheritance_shadow.carbon

@@ -0,0 +1,40 @@
+// 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: Class D
+// CHECK:STDOUT: d.value = 2
+// CHECK:STDOUT: d.GetValue() = 2
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+base class C {
+  fn Method1() {
+    Print("Class C");
+  }
+  fn GetValue[me: Self]() -> i32 {
+    return me.value;
+  }
+  var value: i32;
+}
+
+class D extends C {
+  fn Method1() {
+    Print("Class D");
+  }
+  var value: i32;
+}
+
+fn Main() -> i32 {
+  // Initialize derived value first, base value second
+  var d: D = {.value = 2, .value = 1};
+  d.Method1();
+
+  Print("d.value = {0}", d.value);
+  Print("d.GetValue() = {0}", d.GetValue());
+  return 0;
+}