PR16867, linking object with separate debug file

This teaches the DWARF2 find_line functions how to deal with separate
debug relocatable object files.  Also fixes a major bug:
When _bfd_dwarf2_slurp_debug_info was split out, place_sections ran
after .debug_info was relocated.  This defeated the whole purpose of
place_sections.  See the comment I added before place_sections.
Fixes some minor bugs too:
- place_sections didn't set VMA for alloc but non-load sections (bss).
- zero size sections can have symbols, so they need their VMA set too.
- last_vma was incorrectly adjusted.
- my last change to place_sections left VMA unchanged for .debug_info
  when the linker has mapped input to output sections, but this is
  wrong since bfd_simple_get_relocated_section_contents unmaps debug
  sections.

	PR 16867
	* dwarf2.c: Formatting.
	(struct dwarf2_debug): Make adjusted_section_count signed.
	(unset_sections): Make i signed.
	(set_debug_vma): New function.
	(place_sections): Handle separate debug object file.  Set VMA
	on debug sections, even if they have an output section.  Also
	set VMA on zero size sections, and non-load but alloc sections.
	Set adjusted_section_count to -1 when no section adjustment.
	Malloc adjusted_sections.  Don't double last_vma.  Transfer
	alloc section VMAs to separate debug file.
	(_bfd_dwarf2_cleanup_debug_info): Free adjusted_sections.
	(_bfd_dwarf2_slurp_debug_info): Add do_place parameter.  Drop
	test on symbols being the same before using old stash.  Read
	and use separate debug file symbols.  Call place_sections.
	(find_line): Don't call place_sections here.
	* libbfd-in.h (_bfd_dwarf2_slurp_debug_info): Update proto.
	* libbfd.h: Regenerate.
	* mach-o.c (bfd_mach_o_find_nearest_line): Adjust
	_bfd_dwarf2_slurp_debug_info call.
	* simple.c (simple_save_output_info): Clarify comment.