do not use "1" for line marker for the main file

"1" means entering a new file (from a different one), but the main
file is not included from anything (and this would e.g. confuse -Wunused-macros
to not report unused macros in the main file, see pr15610, or also see pr18948).
The line marker is still useful e.g. if the resulting file is renamed or used
via a pipe.

llvm-svn: 207764