PR22209, invalid memory read in find_abstract_instance_name

PR22209, invalid memory read in find_abstract_instance_name

This patch adds bounds checking for DW_FORM_ref_addr die refs, and
calculates them relative to the first .debug_info section.  See the
big comment for why calculating relative to the current .debug_info
section was wrong for relocatable object files.

	PR 22209
	* dwarf2.c (struct comp_unit): Delete sec_info_ptr field.
	(find_abstract_instance_name): Calculate DW_FORM_ref_addr relative
	to stash->info_ptr_memory, and check die_ref is within that memory.
	Set info_ptr_end correctly when another CU is refd.  Check die_ref
	for DW_FORM_ref4 etc. is within CU.