* cppfiles.c (stack_file): Leave filename as "" rather than "<stdin>".

	* line-map.h (linemap_add): Update comments.
	* line-map.c (linemap_add): Update comments, interpret zero-length
	filename as "<stdin>".


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@69952 138bc75d-0d04-0410-961f-82ee72b054a4