final: accept markers at line 0

final: accept markers at line 0

Back when I introduced debug markers, I seem to have been under the
impression that location line 0 would only ever occur for unknown and
builtin locations.

Though line 0 never comes up in normal processing of source files, and
debug info formats often cannot represent them, I suppose there's no
need to preemptively discard them during final.

for  gcc/ChangeLog

	PR debug/97714
	* final.c (notice_source_line): Narrow down the condition to
	skip a line-0 marker.

for  gcc/testsuite/ChangeLog

	PR debug/97714
	* gcc.dg/debug/pr97714.c: New.