Move signatured_type::type to unshareable object

Move signatured_type::type to unshareable object

signatured_type has a link to the "struct type".  However, types are
inherently objfile-specific, so once sharing is implemented, this will
be incorrect.

This patch moves the type to a new map in the DWARF unshareable
object.

gdb/ChangeLog:

YYYY-MM-DD  Tom Tromey  <tom@tromey.com>
YYYY-MM-DD  Simon Marchi  <simon.marchi@efficios.com>

	* dwarf2/read.h (struct dwarf2_per_objfile)
	<get_type_for_signatured_type, set_type_for_signatured_type>:
	New methods.
	<m_type_map>: New member.
	(struct signatured_type) <type>: Remove.
	* dwarf2/read.c
	(dwarf2_per_objfile::get_type_for_signatured_type,
	dwarf2_per_objfile::set_type_for_signatured_type): New.
	(get_signatured_type): Use new methods.

Change-Id: I765ae3c43fae1064f51ced352167a57638609f02