Add the following sentence as a new final paragraph at the end of Section
"A DW_MACINFO_start_file entry representing the base source file should
encode the value 0 for its line number operand."
We could leave this unspecified or we could specify some other value
or we could specify that any arbitrary value is OK. But I can't imagine why.
The wording is chosen to parallel that for MACINFO command line definitions
at the end of 6.3.3.