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_function_call.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 C, call 0
+// CHECK:STDOUT: Class D, call 0
+// CHECK:STDOUT: Class C, call 1
+// CHECK:STDOUT: Class C, call 2
+// CHECK:STDOUT: Class D, call 2
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+base class C {
+  fn FunctionC(var i: i32) {
+    Print("Class C, call {0}", i);
+  }
+}
+
+class D extends C {
+  fn FunctionD(var i: i32) {
+    C.FunctionC(i);
+    Print("Class D, call {0}", i);
+  }
+}
+
+fn Main() -> i32 {
+  // Calling function from class type
+  D.FunctionD(0);
+  // Note: D.FunctionC(0); is not accessible
+
+  // Calling function from class object
+  var d: D = {};
+  d.FunctionC(1);
+  d.FunctionD(2);
+  return 0;
+}