[libc++] protect absent atomic types with ifdef (#82351)

Otherwise modules.std-module.sh.cpp test fails with following error:
    error: no member named 'atomic_signed_lock_free' in namespace 'std'
when the types are not available.

Author: domin144

libcxx/modules/std/atomic.inc

@@ -111,8 +111,10 @@ export namespace std {
   using std::atomic_uintmax_t;
   using std::atomic_uintptr_t;
 
+#ifndef _LIBCPP_NO_LOCK_FREE_TYPES
   using std::atomic_signed_lock_free;
   using std::atomic_unsigned_lock_free;
+#endif
 
   // [atomics.flag], flag type and operations
   using std::atomic_flag;