[libc++] Add missing include promote.h in <cmath>

[ldionne]

Remove superfluous <cstddef> include in promote.h, that should fix CI.

libcxx/include/__type_traits/promote.h

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___TYPE_TRAITS_PROMOTE_H
 #define _LIBCPP___TYPE_TRAITS_PROMOTE_H
 
 #include <__config>
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_same.h>
 #include <__utility/declval.h>
-#include <cstddef>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <class _Tp>

libcxx/include/cmath

 
 #include <__assert> // all public C++ headers provide the assertion handler
 #include <__config>
 #include <__type_traits/enable_if.h>
 #include <__type_traits/is_arithmetic.h>
 #include <__type_traits/is_constant_evaluated.h>
 #include <__type_traits/is_floating_point.h>
 #include <__type_traits/is_same.h>
+#include <__type_traits/promote.h>
 #include <__type_traits/remove_cv.h>
 #include <version>
 
 #include <math.h>
 
 #ifndef _LIBCPP_MATH_H
 #   error <cmath> tried including <math.h> but didn't find libc++'s <math.h> header. \
           This usually means that your header search paths are not configured properly. \