gdb: pass signature to allocate_signatured_type and signatured_type constructor

gdb: pass signature to allocate_signatured_type and signatured_type constructor

All signatured_type constucted (even those used only for lookups in hash
maps) need a signature.  Enforce that by passing the signature all the
way to the signatured_type constructor.

gdb/ChangeLog:

	* dwarf2/read.h (struct structured_type) <signatured_type>: New.
	Update all callers.
	(struct dwarf2_per_bfd) <allocate_signatured_type>: Add
	signature parameter, update all callers.
	* dwar2/read.c (dwarf2_per_bfd::allocate_signatured_type): Add
	signature parameter.

Change-Id: I99bc1f88f54127666aa133ddbbabb7f7668fa14a