[gn build] Port 7ae61a3

Author: llvmgnsyncbot

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

@@ -833,6 +833,7 @@ if (current_toolchain == default_toolchain) {
       "__type_traits/decay.h",
       "__type_traits/dependent_type.h",
       "__type_traits/desugars_to.h",
+      "__type_traits/detected_or.h",
       "__type_traits/disjunction.h",
       "__type_traits/enable_if.h",
       "__type_traits/extent.h",