Change file_full_name and file_file_name methods

This changes file_full_name and file_file_name methods to be methods
on line_header.  This seems more clear to me.

2020-02-08  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (struct line_header) <file_full_name,
	file_file_name>: Declare methods.
	(dw2_get_file_names_reader): Update.
	(file_file_name): Now a method.
	(file_full_name): Likewise.
	(macro_start_file): Update.

Change-Id: I50d3e91665a9637c732e1e8d8e4263764c766d9c