Track a resolved Member on each member access expression. (#1333)

Type checking now stores information on a member access identifying the member that was accessed, not only its name. This parallels what we do for other similar constructs whose meaning is resolved by name lookup or type-checking, and will allow us to avoid redoing lookups in some cases during interpretation.

explorer/ast/member.cpp

@@ -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
+
+#include "explorer/ast/member.h"
+
+#include "explorer/ast/declaration.h"
+
+namespace Carbon {
+
+Member::Member(Nonnull<const Declaration*> declaration)
+    : member_(declaration) {}
+
+Member::Member(Nonnull<const NamedValue*> struct_member)
+    : member_(struct_member) {}
+
+auto Member::name() const -> std::string_view {
+  if (const Declaration* decl = member_.dyn_cast<const Declaration*>()) {
+    return GetName(*decl).value();
+  } else {
+    return member_.get<const NamedValue*>()->name;
+  }
+}
+
+auto Member::type() const -> const Value& {
+  if (const Declaration* decl = member_.dyn_cast<const Declaration*>()) {
+    return decl->static_type();
+  } else {
+    return *member_.get<const NamedValue*>()->value;
+  }
+}
+
+auto Member::declaration() const -> std::optional<Nonnull<const Declaration*>> {
+  if (const Declaration* decl = member_.dyn_cast<const Declaration*>()) {
+    return decl;
+  }
+  return std::nullopt;
+}
+
+}  // namespace Carbon