Omit atomic_{,un}signed_lock_free if unsupported

On targets that have limited atomic support, e.g. ones that define
ATOMIC_*_LOCK_FREE to '1' ("sometimes lock free"), we would end up
referencing yet-undefined __libcpp_{,un}signed_lock_free.

This commit adds a guard to prevent these references for such
targets.

Differential Revision: https://reviews.llvm.org/D118391

libcxx/include/atomic

@@ -2692,10 +2692,13 @@ typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, char>::typ
 typedef conditional<_LIBCPP_CONTENTION_LOCK_FREE, __cxx_contention_t, unsigned char>::type      __libcpp_unsigned_lock_free;
 #else
     // No signed/unsigned lock-free types
+#define _LIBCPP_NO_LOCK_FREE_TYPES
 #endif
 
+#if !defined(_LIBCPP_NO_LOCK_FREE_TYPES)
 typedef atomic<__libcpp_signed_lock_free> atomic_signed_lock_free;
 typedef atomic<__libcpp_unsigned_lock_free> atomic_unsigned_lock_free;
+#endif
 
 #define ATOMIC_FLAG_INIT {false}
 #define ATOMIC_VAR_INIT(__v) {__v}