The code looks good.

The patch does two things. One is the addition of the column as 3rd argument, as described. The other, as far as I can see, replaces a file number by a file name (literal string) in some places. Why is it correct to do so? Why is it necessary to do so?

The diff makes it hard to see what's going on, the labelled arguments are a bit confusing.
If we reorder lines from the patch to make it more explicit, the only change is adding labels:

 let emit_debug_info dbg =
-  emit_debug_info_gen dbg (fun file_num file_name ->
-    emit_string "\t.file\t";
-    emit_int file_num; emit_char '\t';
-    emit_string_literal file_name; emit_char '\n';
-  )
+  emit_debug_info_gen dbg (fun ~file_num ~file_name ->
+      emit_string "\t.file\t";
+      emit_int file_num; emit_char '\t';
+      emit_string_literal file_name; emit_char '\n';
+    )
-    (fun file_num line ->
-      emit_string "\t.loc\t";
-      emit_int file_num; emit_char '\t';
-      emit_int line; emit_char '\n')
+    (fun ~file_num ~line ~col:_ ->
+       emit_string "\t.loc\t";
+       emit_int file_num; emit_char '\t';
+       emit_int line; emit_char '\n')

The .file directive is only used to associate an id to a file name, the output don't change for "old-style" emitters.

Merged in trunk. Thanks for the patch.

PR#7226 indicates that this breaks build under clang when Location.none is used, with a negative column value.