Refactor BindName to support tracking the enclosing scope. (#3566)

This is a step towards adding enclosing scopes for imports. It creates
an indirection for all bind names.

We discussed specializing for bindings that are in function scope (i.e.,
not a useful enclosing scope for imports or diagnostics). However, the
thought is to go ahead with this singular approach for now, and only
change structure if it's a performance issues so that we have
incrementally fewer instructions to handle.

toolchain/check/handle_function.cpp

@@ -262,8 +262,9 @@ auto HandleFunctionDefinitionStart(Context& context,
     });
 
     if (auto fn_param = param.TryAs<SemIR::BindName>()) {
-      context.AddNameToLookup(fn_param->parse_node, fn_param->name_id,
-                              param_id);
+      context.AddNameToLookup(
+          fn_param->parse_node,
+          context.bind_names().Get(fn_param->bind_name_id).name_id, param_id);
     } else {
       CARBON_FATAL() << "Unexpected kind of parameter in function definition "
                      << param;