[libcxx][clang-modules] Fix headers being marked as textual

libcxx/include/module.modulemap

@@ -1,13 +1,36 @@
 // This module contains headers related to the configuration of the library. These headers
 // are free of any dependency on the rest of libc++.
 module std_config [system] {
-  textual header "__config"
-  textual header "__configuration/abi.h"
-  textual header "__configuration/availability.h"
-  textual header "__configuration/compiler.h"
-  textual header "__configuration/language.h"
-  textual header "__configuration/platform.h"
-  textual header "version"
+  // Each of these submodules has "export *" to preserve compatibility by ensuring that
+  // when you include the module, you get the transitive deps.
+  module config {
+    header "__config"
+    export *
+  }
+  module abi {
+    header "__configuration/abi.h"
+    export *
+  }
+  module availability {
+    header "__configuration/availability.h"
+    export *
+  }
+  module compiler {
+    header "__configuration/compiler.h"
+    export *
+  }
+  module language {
+    header "__configuration/language.h"
+    export *
+  }
+  module platform {
+    header "__configuration/platform.h"
+    export *
+  }
+  module version {
+    header "version"
+    export *
+  }
 }
 
 module std_core [system] {