[libc++][modules] Fixes exporting named declarations.

[Mordante]

Summary

@ChuanqiXu noticed std::atomic was not properly exported in the std module.
Investigation showed other named declarations were not exported either. This
fixes the issue.

Depends on D156550

[Mordante]

Note this patch is a minimal fix intended to be backported to LLVM-17. The tests are in D156594.

libcxx/modules/std/atomic.cppm

export namespace std {
   using std::memory_order_acq_rel;
   using std::memory_order_acquire;
   using std::memory_order_consume;
   using std::memory_order_relaxed;
   using std::memory_order_release;
   using std::memory_order_seq_cst;
 
   using std::kill_dependency;
-} // namespace std
-
-namespace std {
 
   // [atomics.ref.generic], class template atomic_ref
   // [atomics.ref.pointer], partial specialization for pointers
   // using std::atomic_ref;
 
   // [atomics.types.generic], class template atomic
   using std::atomic;
 

libcxx/modules/std/execution.cppm

 
 export module std:execution;
 export namespace std {
   // [execpol.type], execution policy type trait
   using std::is_execution_policy;
   using std::is_execution_policy_v;
 } // namespace std
 
-namespace std::execution {
+export namespace std::execution {
   // [execpol.seq], sequenced execution policy
   using std::execution::sequenced_policy;
 
   // [execpol.par], parallel execution policy
   using std::execution::parallel_policy;
 
   // [execpol.parunseq], parallel and unsequenced execution policy
   using std::execution::parallel_unsequenced_policy;

libcxx/modules/std/filesystem.cppm

export namespace std::filesystem {
   using std::filesystem::weakly_canonical;
 
   // [depr.fs.path.factory]
   using std::filesystem::u8path;
 
 } // namespace std::filesystem
 
 // [fs.path.hash], hash support
-namespace std {
+export namespace std {
   using std::hash;
 }
 
-namespace std::ranges {
+export namespace std::ranges {
   using std::ranges::enable_borrowed_range;
   using std::ranges::enable_view;
 } // namespace std::ranges