libstdc++: Update LWG 4166 changes to concat_view::end() [PR120934]

libstdc++: Update LWG 4166 changes to concat_view::end() [PR120934]

In r15-4555-gf191c830154565 we proactively implemented the initial
proposed resolution for LWG 4166 which later turned out to be
insufficient, since we must also require equality_comparable of the
underlying iterators before concat_view could be a common range.

This patch implements the updated P/R, requiring all underlying
iterators to be forward (which implies equality_comparable) before
making concat_view common, which fixes the testcase from this PR.

	PR libstdc++/120934

libstdc++-v3/ChangeLog:

	* include/std/ranges (concat_view::end): Refine condition
	for returning an iterator instead of default_sentinel as
	per the updated P/R for LWG 4166.
	* testsuite/std/ranges/concat/1.cc (test05): New test.

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