Explorer: move subtyping logic to TypeChecker (#2484)

Addresses comments from this discussion: https://github.com/carbon-language/carbon-lang/pull/2460#discussion_r1046444433

Features:
* Move subtyping logic from Interpreter to TypeChecker, exposing subtyping as a series of access to `.base`.
    * Excludes function parameter conversion, which is still done in ::Convert due to parameters conversion being handled differently.

Changes:
* Add new `class BaseAccessExpression : public MemberAccessExpression`, allowing rewrites
* Handle `BaseAccessExpression` expression type in Interpreter
* Move subtyping logic to `TypeChecker::ImplicitlyConvert`

explorer/testdata/class/class_subtyping_basic.carbon

@@ -0,0 +1,31 @@
+// 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).a: 1
+// CHECK:STDOUT: Foo(&d): 1
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+base class C {
+  var a: i32;
+}
+
+class D extends C {
+}
+
+fn Foo(c: C*) -> i32 {
+  return (*c).a;
+}
+
+fn Main() -> i32 {
+  var d: D = { .base = {.a = 1} };
+  var c: C* = &d;
+  Print("(*c).a: {0}", (*c).a);
+  Print("Foo(&d): {0}", Foo(&d));
+  return 0;
+}