dwarf2read: C++ify dwo_file

dwarf2read: C++ify dwo_file

This patch changes dwo_file to be allocated/deallocated with new/delete,
so that we can start using C++ features in it, and in struct
dwo_sections.

The free_dwo_file function becomes the destructor of struct dwo_file
(and will disappear in upcoming patches, which will use gdb_bfd_ref_ptr
for dbfd and an std::vector for sections.types).

gdb/ChangeLog:

	* dwarf2read.h (struct dwarf2_per_objfile) <dwo_files>: Change
	type to htab_up.
	* dwarf2read.c (struct dwo_file): Initialize fields.
	<~dwo_file>: New.
	(free_dwo_file): Remove, move content to ~dwo_file.
	(struct dwo_file_deleter): Remove.
	(dwo_file_up>: Remove custom deleter.
	(free_dwo_files): Remove.
	(dwarf2_per_objfile::~dwarf2_per_objfile): Don't explicitly free
	dwo_files.
	(process_skeletonless_type_units): Call unique_ptr::get.
	(allocate_dwo_file_hash_table): Add deleter to created hash
	table.  Change return type to htab_up.
	(lookup_dwo_file_slot): Don't memset dwo_file, call
	unique_ptr::get.
	(create_dwo_unit_in_dwp_v1): Allocate dwo_file with new.
	(create_dwo_unit_in_dwp_v2): Likewise.
	(open_and_init_dwo_file): Likewise.
	(free_dwo_file_from_slot): Remove.