Revert "Update aosp/master libcxx rebase to r263688"

The world is burning.

This reverts commit c004fd909c, reversing
changes made to 1418e4163d.

include/module.modulemap

@@ -455,13 +455,9 @@ module std [system] {
     export *
   }
 
-  // FIXME: We don't have modules for the <foo.h> headers, because they might
-  // be included from the C library's headers, and that would create a #include
-  // cycle. For the same reason, we don't have a module for __config.
-  //module __config { header "__config" export * }
-
   // FIXME: These should be private.
   module __bit_reference { header "__bit_reference" export * }
+  module __config { header "__config" export * }
   module __debug { header "__debug" export * }
   module __functional_base { header "__functional_base" export * }
   module __hash_table { header "__hash_table" export * }