C++ify dwarf2_per_objfile

C++ify dwarf2_per_objfile

This makes dwarf2_per_objfile a class with cdtors.

A following patch will add a non-trivial field to struct
dwarf2_per_objfile, making dwarf2_per_objfile itself non-trivial.
Since dwarf2_per_objfile is allocated in an obstack, we need to run
its cdtors manually.

Tested on x86-64 GNU/Linux.

gdb/ChangeLog:
2017-07-17  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (dwarf2_per_objfile): In-class initialize all
	fields.
	(dwarf2_per_objfile::dwarf2_per_objfile(objfile*, const
	dwarf2_debug_sections*)): New.
	(dwarf2_per_objfile::dwarf2_per_objfile(const
	dwarf2_per_objfile&)): Declare as deleted.
	(dwarf2_per_objfile::operator=): Declare as deleted.
	(dwarf2_per_objfile::dwarf2_per_objfile)
	(dwarf2_per_objfile::~dwarf2_per_objfile)
	(dwarf2_per_objfile::free_cached_comp_units): New.
	(dwarf2_has_info): dwarf2_per_objfile initialization code moved to
	ctor.  Call dwarf2_per_objfile's ctor manually.
	(dwarf2_locate_sections): Deleted/refactored as ...
	(dwarf2_per_objfile::locate_sections): ... this new method.
	(free_cached_comp_units): Defer to
	dwarf2_per_objfile::free_cached_comp_units.
	(dwarf2_free_objfile): Call dwarf2_per_objfile's dtor manually.