New class allocate_on_obstack

New class allocate_on_obstack

This patch adds a new class allocate_on_obstack, and let dwarf2_per_objfile
inherit it, so that dwarf2_per_objfile is automatically allocated on
obstack, and "delete dwarf2_per_objfile" doesn't de-allocate any space.

gdb:

2018-02-16  Yao Qi  <yao.qi@linaro.org>

	* block.c (block_namespace_info): Inherit allocate_on_obstack.
	(block_initialize_namespace): Use new.
	* dwarf2read.c (dwarf2_per_objfile): Inherit allocate_on_obstack.
	(dwarf2_free_objfile): Use delete.
	* gdbtypes.c (type_pair): Inherit allocate_on_obstack.
	(copy_type_recursive): Use new.
	* gdb_obstack.h (allocate_on_obstack): New.