Change dwarf2_per_objfile::quick_file_names_table to htab_up

This changes dwarf2_per_objfile::quick_file_names_table to be an
htab_up.  This just removes a bit of manual management.

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

	* dwarf2/read.c (~dwarf2_per_objfile): Update.
	(create_quick_file_names_table): Return htab_up.
	(dw2_get_file_names_reader, dw2_forget_cached_source_info):
	Update.
	* dwarf2/read.h (struct dwarf2_per_objfile)
	<quick_file_names_table>: Now htab_up.

Change-Id: I4ff2fce8b8af27f4bfe01a11b97a889edfd23151