libstdc++: Fix ranges::shuffle for non-sized range [PR121917]

libstdc++: Fix ranges::shuffle for non-sized range [PR121917]

ranges::shuffle has a two-at-a-time PRNG optimization (copied from
std::shuffle) that considers the PRNG width vs the size of the range.
But in C++20 a random access sentinel isn't always sized so we can't
unconditionally do __last - __first to obtain the size in constant
time.

We could instead use ranges::distance, but that'd take linear time for a
non-sized sentinel which makes the optimization less clear of a win.  So
this patch instead makes us only consider this optimization for sized
ranges.

	PR libstdc++/121917

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algo.h (__shuffle_fn::operator()): Only
	consider the two-at-a-time PRNG optimization if the range is
	sized.
	* testsuite/25_algorithms/shuffle/constrained.cc (test03): New
	test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>