...like other preprocessors do.

Observe:

$ cat ATTIC/backtrace.groff
.\" A comment line.
.backtrace
.lf 11 filename.groff
.backtrace
$ nroff ATTIC/backtrace.groff
troff: backtrace: file 'ATTIC/backtrace.groff':2
troff: backtrace: file 'filename.groff':11
$ ./build/test-groff -g -T ascii ATTIC/backtrace.groff
troff: backtrace: file '<standard input>':2
troff: backtrace: file 'filename.groff':11

Because, since 1.23.0, the groff `-I` option now implies `-g`, this missing feature is more noticeable than it used to be, for instance when "doc/grnexampl.me" produces diagnostics.

This shouldn't be too hard to implement.  libgroff already provides support facilities, and our other preprocessors use them.

$ git grep interpret_lf_args src
src/include/lf.h:bool interpret_lf_args(const char *p);
src/libs/libgroff/lf.cpp:bool interpret_lf_args(const char *p)
src/preproc/eqn/main.cpp:      if (interpret_lf_args(linebuf.contents() + 3))
src/preproc/pic/main.cpp:       interpret_lf_args(line.contents());
src/preproc/refer/refer.cpp:      if (interpret_lf_args(line.contents() + 3))
src/preproc/soelim/soelim.cpp:  interpret_lf_args(line.contents());
src/preproc/tbl/main.cpp:       interpret_lf_args(line.contents());
src/preproc/tbl/main.cpp:                   interpret_lf_args(args.contents());
src/preproc/tbl/main.cpp:         interpret_lf_args(line.contents() + 3);
$ git grep normalize_for_lf src
src/include/lf.h:void normalize_for_lf (string &fn);
src/libs/libgroff/lf.cpp:void normalize_for_lf (string &fn)
src/libs/libgroff/lf.cpp:void normalize_for_lf (string &)
src/preproc/eqn/main.cpp:  normalize_for_lf(fn);
src/preproc/pic/main.cpp:  normalize_for_lf(fn);
src/preproc/preconv/preconv.cpp:    normalize_for_lf(fn);
src/preproc/refer/refer.cpp:  normalize_for_lf(fn);
src/preproc/soelim/soelim.cpp:  normalize_for_lf(whole_filename);
src/preproc/tbl/main.cpp:         normalize_for_lf(fn);
src/preproc/tbl/table.cpp:    normalize_for_lf(filename);