Change line_table methods to return unique_xmalloc_ptr

Change line_table methods to return unique_xmalloc_ptr

This changes the two new line_table methods to return
unique_xmalloc_ptr.  This removes a bit of manual memory management.

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

	* dwarf2/read.c (struct line_header) <file_full_name,
	file_file_name>: Return unique_xmalloc_ptr.
	(line_header::file_file_name): Update.
	(line_header::file_full_name): Update.
	(dw2_get_file_names_reader): Update.
	(macro_start_file): Update.

Change-Id: I9442dba43882fb26097d0770a291eea2b03913a4