Change dwarf2_per_objfile::type_unit_groups to htab_up

Change dwarf2_per_objfile::type_unit_groups to htab_up

This changes dwarf2_per_objfile::type_unit_groups to be an htab_up,
again allowing us to move the memory used by the hash table from the
objfile obstack to the heap.

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

	* dwarf2/read.c (allocate_type_unit_groups_table): Return
	htab_up.  Don't allocate on obstack.
	(get_type_unit_group, dwarf2_build_psymtabs_hard): Update.
	* dwarf2/read.h (struct dwarf2_per_objfile) <type_unit_groups>:
	Change type to htab_up.

Change-Id: Ia045df0ff3ec30aac813da5a9a2314a607ef7ec8