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 83cae6c4)