Explorer: support virtual class methods (#2462)

Features:
* Add `virtual` virtual override keyword for functions
* Support `virtual` class methods using dynamic dispatch

Changes:
* Add `vtable` in `NominalClassType`, 
* Add `NominalClassValue**` in class values pointing to  descendant-most class
* Resolve virtual methods during member lookup

Limitations:
* Does not include yet `impl`, `abstract` virtual override keywords, or the complete logic for virtual function declaration

Depends on #2460 
Relates to #1881 
Relates to #2493

explorer/testdata/class/virtual_method_self.carbon

@@ -0,0 +1,53 @@
+// 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: c.Foo(): 1
+// CHECK:STDOUT: d.Foo(): 2
+// CHECK:STDOUT: e.Foo(): 3
+// CHECK:STDOUT: (*dp).Foo(): 3
+// CHECK:STDOUT: (*dc).Foo(): 3
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+base class C {
+  var value_c: i32;
+  virtual fn Foo[self: Self]() -> i32 {
+    return self.value_c;
+  }
+}
+
+base class D extends C {
+  var value_d: i32;
+  // TODO: This should instead use `impl` when supported
+  virtual fn Foo[self: Self]() -> i32 {
+    return self.value_d;
+  }
+}
+
+class E extends D {
+  var value_e: i32;
+  // TODO: This should instead use `impl` when supported
+  virtual fn Foo[self: Self]() -> i32 {
+    return self.value_e;
+  }
+}
+
+fn Main() -> i32 {
+  var c: C = {.value_c = 1};
+  Print("c.Foo(): {0}", c.Foo());
+  var d: D = {.value_d = 2, .base = {.value_c = 1}};
+  Print("d.Foo(): {0}", d.Foo());
+  var e: E = {.value_e = 3, .base={.value_d = 2, .base = {.value_c = 1}}};
+  Print("e.Foo(): {0}", e.Foo());
+  var dp: D* = &e;
+  Print("(*dp).Foo(): {0}", (*dp).Foo());
+  var dc: C* = &e;
+  Print("(*dc).Foo(): {0}", (*dc).Foo());
+
+  return 0;
+}