[libc++] Disable local submodule visibility in the modules build

Differential Revision: https://reviews.llvm.org/D119468

NOKEYCHECK=True
GitOrigin-RevId: 73382278825d2612bbcd59b5627a82189fa4ae11
diff --git a/include/module.modulemap b/include/module.modulemap
index 5c21e5d..6a55bf3 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -2,8 +2,9 @@
 // since __config may be included from C headers which may create an
 // include cycle.
 module std_config [system] [extern_c] {
-  textual header "__config"
-  textual header "__config_site"
+  header "__config"
+  header "__config_site"
+  export *
 }
 
 module std [system] {