Automerge: [gn build] Port d2521f1

Author: llvmgnsyncbot

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

@@ -402,6 +402,8 @@ if (current_toolchain == default_toolchain) {
       "__configuration/abi.h",
       "__configuration/availability.h",
       "__configuration/compiler.h",
+      "__configuration/experimental.h",
+      "__configuration/hardening.h",
       "__configuration/language.h",
       "__configuration/platform.h",
       "__coroutine/coroutine_handle.h",