[libc++] Remove unnecessary includes from <atomic> (llvm#82880)

This reduces the include time of `<atomic>` from 135ms to 88ms.

libcxx/include/__atomic/aliases.h

@@ -18,7 +18,6 @@
 #include <__type_traits/make_unsigned.h>
 #include <cstddef>
 #include <cstdint>
-#include <cstdlib>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header

libcxx/include/__config

@@ -460,6 +460,11 @@ _LIBCPP_HARDENING_MODE_DEBUG
 #    define __has_constexpr_builtin(x) 0
 #  endif
 
+// This checks wheter a Clang module is built
+#  ifndef __building_module
+#    define __building_module(...) 0
+#  endif
+
 // '__is_identifier' returns '0' if '__x' is a reserved identifier provided by
 // the compiler and '1' otherwise.
 #  ifndef __is_identifier

libcxx/include/__thread/support/pthread.h

@@ -30,7 +30,10 @@
 // so libc++'s <math.h> usually absorbs atomic_wide_counter.h into the
 // module with <math.h> and makes atomic_wide_counter.h invisible.
 // Include <math.h> here to work around that.
-#include <math.h>
+// This checks wheter a Clang module is built
+#if __building_module(std)
+#  include <math.h>
+#endif
 
 #ifndef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
 #  pragma GCC system_header

libcxx/include/atomic

@@ -620,6 +620,7 @@ template <class T>
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <cmath>
 #  include <compare>
+#  include <cstdlib>
 #  include <cstring>
 #  include <type_traits>
 #endif

libcxx/test/libcxx/transitive_includes/cxx23.csv

@@ -30,7 +30,6 @@ array stdexcept
 array version
 atomic cstddef
 atomic cstdint
-atomic cstdlib
 atomic cstring
 atomic ctime
 atomic limits

libcxx/test/libcxx/transitive_includes/cxx26.csv

@@ -30,7 +30,6 @@ array stdexcept
 array version
 atomic cstddef
 atomic cstdint
-atomic cstdlib
 atomic cstring
 atomic ctime
 atomic limits