Convert dwarf2_per_objfile to type-safe registry API

Convert dwarf2_per_objfile to type-safe registry API

This changes dwarf2_per_objfile to use the type-safe registry API.
This also changes dwarf2_per_objfile not to be allocated on an
obstack.  It seemed clearer to me to simply allocate it on the heap;
and I didn't see a drawback from doing so.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tom@tromey.com>

	* dwarf2read.h (struct dwarf2_per_objfile): Don't inherit from
	allocate_on_obstack.
	* dwarf2read.c (dwarf2_objfile_data_key): Change type.
	(get_dwarf2_per_objfile): Update.
	(set_dwarf2_per_objfile): Remove.
	(dwarf2_has_info, dwarf2_get_section_info): Update.
	(dwarf2_free_objfile): Remove.
	(_initialize_dwarf2_read): Update.