Don't re-export top-level modules.

[v.g.vassilev]

Summary

In https://reviews.llvm.org/D119036 we fixed some of the infrastructure by removing the textual keyword.

The underlying issue of PR50592 was that clang can re-export only submodules but under some conditions we needed to re-export the standalone module std_config via std.

This patch provides a better fix to the symptom D119036 fixed.

libcxx/include/module.modulemap.in

 // define the module for __config outside of the top level 'std' module
 // since __config may be included from C headers which may create an
 // include cycle.
 module std_config [system] [extern_c] {
-  header "__config"
+  textual header "__config"
   export *
 }
 
 module std [system] {
-  export std_config
   // FIXME: The standard does not require that each of these submodules
   // re-exports its imported modules. We should provide an alternative form of
   // export that issues a warning if a name from the submodule is used, and
   // use that to provide a 'strict mode' for libc++.
 
   // Deprecated C-compatibility headers. These can all be included from within
   // an 'extern "C"' context.
   module depr [extern_c] {