[gn build] Port 6a66467

Author: llvmgnsyncbot

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -904,6 +904,7 @@ if (current_toolchain == default_toolchain) {
       "__type_traits/void_t.h",
       "__undef_macros",
       "__utility/as_const.h",
+      "__utility/as_lvalue.h",
       "__utility/auto_cast.h",
       "__utility/cmp.h",
       "__utility/convert_to_integral.h",