libstdc++, testsuite: Implement C++26 P3450R1: Extend std::is_within_lifetime paper

Now that r17-4186 fixed constexpr evaluation of static_cast downcasts to diagnose
UB, this patch implements the C++26 P3450R1: Extend std::is_within_lifetime paper
purely in library by using __builtin_constant_p to verify if the static_cast
mandated by the standard is a constant expression.

2026-09-14  Jakub Jelinek  <jakub@redhat.com>

libstdc++-v3/
	* include/bits/version.def (within_lifetime): Change value from 202306
	to 202603.
	* include/bits/version.h: Regenerate.
	* include/std/type_traits: Implement C++26 P3450R1: Extend
	std::is_within_lifetime paper.
	(std::is_within_lifetime): Add _Up template argument defaulted to void
	and verify static_cast<const volatile _Up*>(__p) is a constant
	expression.
gcc/testsuite/
	* g++.dg/cpp26/within-lifetime1.C: Fix up C++26 paper number and name.
	* g++.dg/cpp26/within-lifetime2.C: Likewise.
	* g++.dg/cpp26/within-lifetime3.C: Likewise.
	* g++.dg/cpp26/within-lifetime4.C: Likewise.
	* g++.dg/cpp26/within-lifetime5.C: Likewise.
	* g++.dg/cpp26/within-lifetime6.C: Likewise.
	* g++.dg/cpp26/within-lifetime7.C: Likewise.
	* g++.dg/cpp26/within-lifetime8.C: Likewise.
	* g++.dg/cpp26/within-lifetime9.C: Likewise.
	* g++.dg/cpp26/within-lifetime10.C: New test.
	* g++.dg/cpp26/within-lifetime11.C: New test.
	* g++.dg/cpp26/within-lifetime12.C: New test.

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