Change dwp_file to use htab_up

This changes dwp_file to use htab_up for the loaded_cus and loaded_tus
members.  This lets us avoid allocating the contents of these hash
tables on the objfile obstack.

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

	* dwarf2/read.c (struct dwp_file) <loaded_cus, loaded_tus>: Now
	htab_up.
	(lookup_dwo_unit_in_dwp): Update.
	(allocate_dwp_loaded_cutus_table): Return htab_up.  Don't allocate
	on obstack.

Change-Id: Id61209bf5c6c6faa0c067195af31fbcf26813a3a