Allocate dwp_file with new

Allocate dwp_file with new

This adds a constructor and initializer to dwp_file and changes it to
be allocated with "new".  This removes a bit of manual refcount
management.

Tested by the buildbot.

gdb/ChangeLog
2018-05-18  Tom Tromey  <tom@tromey.com>

	* dwarf2read.h (struct dwarf2_per_objfile) <dwp_file>: Now a
	unique_ptr.
	* dwarf2read.c (struct dwp_file): Add constructor and
	initializers.
	(open_and_init_dwp_file): Return a unique_ptr.
	(dwarf2_per_objfile, create_dwp_hash_table)
	(create_dwo_unit_in_dwp_v1, create_dwo_unit_in_dwp_v2)
	(lookup_dwo_unit_in_dwp): Update.
	(open_and_init_dwp_file, get_dwp_file): Update.