libstdc++: Implement LWG 4166 changes to concat_view::end()

libstdc++: Implement LWG 4166 changes to concat_view::end()

This patch proactively implements the proposed resolution for this LWG
issue, which seems straightforward and slated to get approved as-is.

(No _GLIBCXX_RESOLVE_LIB_DEFECTS code comment is added since concat_view
is C++26, so this isn't a defect against a published standard.)

libstdc++-v3/ChangeLog:

	* include/std/ranges (concat_view::begin): Add space after
	'requires' starting a requires-clause.
	(concat_view::end): Likewise.  Refine condition for returning an
	iterator rather than default_sentinel as per LWG 4166.
	* testsuite/std/ranges/concat/1.cc (test03): Verify LWG 4166
	example.

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