[gn build] Port cb8a0b0

Author: llvmgnsyncbot

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

@@ -420,8 +420,8 @@ if (current_toolchain == default_toolchain) {
       "__tuple",
       "__undef_macros",
       "__utility/as_const.h",
+      "__utility/auto_cast.h",
       "__utility/cmp.h",
-      "__utility/decay_copy.h",
       "__utility/declval.h",
       "__utility/exchange.h",
       "__utility/forward.h",