Add ExternDecl and ExternType for extern classes. (#3893)

This expands `extern` handling for most cases.

toolchain/check/handle_function.cpp

@@ -118,6 +118,8 @@ static auto BuildFunctionDecl(Context& context,
   auto function_decl = SemIR::FunctionDecl{
       context.GetBuiltinType(SemIR::BuiltinKind::FunctionType),
       SemIR::FunctionId::Invalid, decl_block_id};
+  // TODO: Should Function replace is_extern with ExternDecl, similar to
+  // ClassDecl?
   auto function_info = SemIR::Function{
       .name_id = name_context.name_id_for_new_inst(),
       .enclosing_scope_id = name_context.enclosing_scope_id_for_new_inst(),