[libc++] Fix use of static in constexpr (#160180)

`static` is not allowed inside constexpr functions in C++ versions below
23. This is causing a build error when compiled with GCC and warning
when compiled with Clang. This patch fixes this.

---------

Co-authored-by: A. Jiang <[email protected]>

Author: Prabhuk

libcxx/include/__algorithm/make_heap.h

@@ -36,7 +36,7 @@ __make_heap(_RandomAccessIterator __first, _RandomAccessIterator __last, _Compar
   using __diff_t     = __iter_diff_t<_RandomAccessIterator>;
   const __diff_t __n = __last - __first;
 
-  static const bool __assume_both_children = is_arithmetic<__iter_value_type<_RandomAccessIterator> >::value;
+  const bool __assume_both_children = is_arithmetic<__iter_value_type<_RandomAccessIterator> >::value;
 
   // While it would be correct to always assume we have both children, in practice we observed this to be a performance
   // improvement only for arithmetic types.