[libc++] protect absent atomic types with ifdef

[domin144]

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.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Dominik Wójt (domin144)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/82351.diff

1 Files Affected:

• (modified) libcxx/modules/std/atomic.inc (+2)

diff --git a/libcxx/modules/std/atomic.inc b/libcxx/modules/std/atomic.inc
index 88b31ccdb20840..2b54cef863e571 100644
--- a/libcxx/modules/std/atomic.inc
+++ b/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;

[domin144]

@ldionne @mordante
(I can't request review through github, yet.)