libstdc++: perfectly forward std::ranges::clamp arguments

As reported in PR118185, std::ranges::clamp does not correctly forward
the projected value to the comparator. Add the missing forward.

libstdc++-v3/ChangeLog:

	PR libstdc++/118185
	PR libstdc++/100249
	* include/bits/ranges_algo.h (__clamp_fn): Correctly forward the
	projected value to the comparator.
	* testsuite/25_algorithms/clamp/118185.cc: New test.

Signed-off-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>