c++, testsuite: Add tests for C++26 P4136R2 - #line is not in line with existing implementation [PR125842]

IMHO we used to conform even before this paper, pedwarning for #line outside
of [1,2147483647] range and always warning for [4294967296,inf] range,
so we conform even when program with #line outside of [1,2147483647]
range is conditionally supported instead of ill-formed.
Our conditional-support is that we accept [0,0][2147483648,4294967295]
as an extension unless pedantic.

2026-08-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/125842
	* g++.dg/cpp/line-1.C: New test.
	* g++.dg/cpp/line-2.C: New test.
	* g++.dg/cpp/line-3.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>