[gn build] Set _LIBCPP_ABI_NAMESPACE to __$libcxx_abi_version

Matches CMake build.
Otherwise currently everything is literally std::_LIBCPP_ABI_NAMESPACE::foo.

diff --git a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn b/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
index 1118d79..01c7565 100644 (file)
--- a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
+++ b/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
@@ -47,7 +47,7 @@ if (current_toolchain == default_toolchain) {
     if (libcxx_abi_namespace != "") {
       values += [ "_LIBCPP_ABI_NAMESPACE=$libcxx_abi_namespace" ]
     } else {
-      values += [ "_LIBCPP_ABI_NAMESPACE=" ]
+      values += [ "_LIBCPP_ABI_NAMESPACE=__" + libcxx_abi_version ]
     }
   }