When deciding whether to perform the memset optimization in ranges::fill_n, we
were crucially neglecting to check that the output pointer's value type is a
byte type.  This patch adds such a check to the problematic condition in
ranges::fill_n.

At the same time, this patch relaxes the overly conservative
__is_byte<_Tp>::__value check that requires the fill type be a byte type.  It's
overly conservative because it means we won't enable the memset optimization in
the following example

  char c[100];
  ranges::fill(c, 37);

because the fill type is deduced to be int here.  Rather than requiring that the
fill type be a byte type, it seems safe to just require the fill type be an
integral type, which is what this patch does.

libstdc++-v3/ChangeLog:

	PR libstdc++/94017
	* include/bits/ranges_algobase.h (__fill_n_fn::operator()): Refine
	condition for when to use memset, making sure to additionally check that
	the output pointer's value type is a non-volatile byte type.  Instead of
	requiring that the fill type is a byte type, just require that it's an
	integral type.
	* testsuite/20_util/specialized_algorithms/uninitialized_fill/94017.cc:
	New test.
	* testsuite/20_util/specialized_algorithms/uninitialized_fill_n/94017.cc:
	New test.
	* testsuite/25_algorithms/fill/94013.cc: Uncomment part that was blocked
	by PR 94017.
	* testsuite/25_algorithms/fill/94017.cc: New test.
	* testsuite/25_algorithms/fill_n/94017.cc: New test.