stage2: add the .debug_line header and associated data types

 * the .debug_line header is written properly
 * link.File.Elf gains:
   - SrcFn, which is now a field in Module.Fn
   - SrcFile, which is now a field in Module.Scope.File
 * link.File.Elf gets a whole *Package field rather than only
   root_src_dir_path.
 * the fields first_dbg_line_file and last_dbg_line_file tell where the
   Line Number Program begins and ends, which alows moving files when
   the header gets too big, and allows appending files to the end.
 * codegen is passed a buffer for emitting .debug_line
   Line Number Program opcodes for functions.

See #5963

There is some work-in-progress code here, but I need to go make some
experimental changes to changing how to represent source locations and I
want to do that in a separate commit.

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