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/fuzzing/ast_to_proto.cpp

@@ -110,6 +110,7 @@ static auto ExpressionToProto(const Expression& expression)
     -> Fuzzing::Expression {
   Fuzzing::Expression expression_proto;
   switch (expression.kind()) {
+    case ExpressionKind::BaseAccessExpression:
     case ExpressionKind::ValueLiteral: {
       // This does not correspond to source syntax.
       break;