The overload for byte types uses memset and isn't constexpr. This adds
the specifier and uses std::is_constant_evaluated() to provide a
compile-time alternative.

Backport from mainline
2020-05-03  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/94933
	* include/bits/stl_algobase.h (__fill_a1): Make overload for byte types
	usable in constant expressions.
	* testsuite/25_algorithms/fill_n/constexpr.cc: Test with bytes and
	non-scalars.