Add a Field node for fields in a class. (#3332)

This replaces the use of `VarStorage` in this case.

Add an `UnboundFieldType` type as the type of a field, in cases where
it's referenced without an accompanying object.

Add a `BindName` node to describe the name binding performed for both
variables and fields so that we can handle them more uniformly.

toolchain/check/handle_file.cpp

@@ -0,0 +1,21 @@
+// 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 "toolchain/check/context.h"
+
+namespace Carbon::Check {
+
+auto HandleFileStart(Context& /*context*/, Parse::Node /*parse_node*/) -> bool {
+  // No action to perform.
+  // TODO: We may want to push `FileStart` as a sentinel so that `Peek`s can't
+  // fail.
+  return true;
+}
+
+auto HandleFileEnd(Context& /*context*/, Parse::Node /*parse_node*/) -> bool {
+  // No action to perform.
+  return true;
+}
+
+}  // namespace Carbon::Check