Move die_type_hash to dwarf2_per_objfile

Move die_type_hash to dwarf2_per_objfile

The die_type_hash field can't be shared between multiple obfiles, as it
holds `struct type` objects, which are objfile-specific.  Move it from
dwarf2_per_bfd to dwarf2_per_objfile and update all references.

gdb/ChangeLog:

	* dwarf2/read.h (struct dwarf2_per_bfd) <die_type_hash>: Move to
	struct dwarf2_per_objfile.
	(struct dwarf2_per_objfile) <die_type_hash>: Move from struct
	dwarf2_per_bfd.
	* dwarf2/read.c (set_die_type): Update.
	(get_die_type_at_offset): Update.

Change-Id: I3589777ed3579bcabafd2ba859d27babe4502bfb