objfile_per_bfd_storage non-POD

A following patch will want to add a std::vector to
objfile_per_bfd_storage.  That makes it non-trivially
constructible/destructible.  Since objfile_per_bfd_storage objects are
allocated on an obstack, we need to call their ctors/dtors manually.
This is what this patch does.  And then since we can now rely on
ctors/dtors being run, make objfile_per_bfd_storage::storage_obstack
be an auto_obstack.

gdb/ChangeLog:
2017-06-27  Pedro Alves  <palves@redhat.com>

	* objfiles.c (get_objfile_bfd_data): Call bfd_alloc instead of
	bfd_zalloc.  Call objfile_per_bfd_storage's ctor.
	(free_objfile_per_bfd_storage): Call objfile_per_bfd_storage's
	dtor.
	* objfiles.h (objfile_per_bfd_storage): Add ctor.  Make
	'storage_obstack' field an auto_obstack.  In-class initialize all
	non-bitfield fields.  Make minsyms_read bool.
	* symfile.c (read_symbols): Adjust.