Make some line_header methods const

Make some line_header methods const

This changes a few line_header methods to be const.  In some cases, a
const overload is added.

gdb/ChangeLog
2020-03-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/line-header.h (struct line_header) <is_valid_file_index,
	file_names_size, file_full_name, file_file_name>: Use const.
	<file_name_at, file_names>: Add const overload.
	* dwarf2/line-header.c (line_header::file_file_name)
	(line_header::file_full_name): Update.