libstdc++: Fix std::__uninitialized_default_n for constant evaluation [PR110542]

libstdc++-v3/ChangeLog:

	PR libstdc++/110542
	* include/bits/stl_uninitialized.h (__uninitialized_default_n):
	Do not use std::fill_n during constant evaluation.

(cherry picked from commit 83cae6c4b788544635a71748e1881c150f42efef)

libstdc++-v3/include/bits/stl_uninitialized.h

@@ -695,6 +695,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline _ForwardIterator
     __uninitialized_default_n(_ForwardIterator __first, _Size __n)
     {
+#ifdef __cpp_lib_is_constant_evaluated
+      if (std::is_constant_evaluated())
+	return __uninitialized_default_n_1<false>::
+		 __uninit_default_n(__first, __n);
+#endif
+
       typedef typename iterator_traits<_ForwardIterator>::value_type
 	_ValueType;
       // See uninitialized_fill_n for the conditions for using std::fill_n.