Split dwarf2_per_objfile into dwarf2_per_objfile and dwarf2_per_bfd

Split dwarf2_per_objfile into dwarf2_per_objfile and dwarf2_per_bfd

This is the first step of splitting dwarf2_per_objfile in two, one
structure for objfile-independent data (dwarf2_per_bfd) and one for
objfile-dependent data (dwarf2_per_objfile).

The existing dwarf2_per_objfile is renamed dwarf2_per_bfd, and a new
dwarf2_per_objfile type is introduced, which sits "in between" the
objfile and dwarf2_per_bfd.

So where we had this before:

  objfile -> dwarf2_per_objfile (*)

we now have this:

  objfile -> dwarf2_per_objfile -> dwarf2_per_bfd (*)

(*) Note that the dwarf2_per_objfile in the former corresponds to
the dwarf2_per_bfd in the latter.

I've done the minimal amount of changes in this patch: following patches
will incrementally move things that are not actually shareable between
objfiles from dwarf2_per_bfd to dwarf2_per_objfile.

Most references to dwarf2_per_objfile objects are changed to
dwarf2_per_objfile->per_bfd.  To avoid many of these replacements, which
would have to be reverted later anyway, I've moved right away the
objfile backlink to the new dwarf2_per_objfile structure in this patch.
I've also moved the read_line_string method, since it references the
objfile backlink, and it's actually not difficult to move.

Once the moves are completed, multiple dwarf2_per_objfile sharing the
same BFD will point to the same single instance of dwarf2_per_bfd (as
long as they don't require relocation).

dwarf2_has_info, where we create these objects, is updated to the new
architecture.

I've had to change the get_gdb_index_contents_ftype typedef and related
functions.  The parameter type was changed from dwarf2_per_objfile to
dwarf2_per_bfd, otherwise the template wouldn't work.

Please excuse the terse ChangeLog entry, I have not listed all the
functions where dwarf2_per_objfile has been changed to
dwarf2_per_objfile->per_bfd.  It would take a considerable amount of
time and would not really be useful in the end.

gdb/ChangeLog:

	* dwarf2/read.h (dwarf2_per_objfile): Rename to dwarf2_per_bfd,
	then introduce a new dwarf2_per_objfile type.
	<read_line_string>: Move to the new dwarf2_per_objfile type.
	<objfile>: Likewise.
	(dwarf2_per_bfd): Rename dwarf2_per_objfile to this.
	* dwarf2/read.c: Replace references to dwarf2_per_objfile with
	dwarf2_per_objfile->per_bfd.
	(dwarf2_per_objfile::dwarf2_per_objfile): Rename to...
	(dwarf2_per_bfd::dwarf2_per_bfd): ... this.
	(dwarf2_per_objfile::free_cached_comp_units): Rename to...
	(dwarf2_per_bfd::free_cached_comp_units): ... this.
	(dwarf2_has_info): Allocate dwarf2_per_bfd.
	(dwarf2_per_objfile::locate_sections): Rename to...
	(dwarf2_per_bfd::locate_sections): ... this.
	(dwarf2_per_objfile::get_cutu): Rename to...
	(dwarf2_per_bfd::get_cutu): ... this.
	(dwarf2_per_objfile::get_cu): Rename to...
	(dwarf2_per_bfd::get_cu): ... this.
	(dwarf2_per_objfile::get_tu): Rename to...
	(dwarf2_per_bfd::get_tu): ... this.
	(dwarf2_per_objfile::allocate_per_cu): Rename to...
	(dwarf2_per_bfd::allocate_per_cu): ... this.
	(dwarf2_per_objfile::allocate_signatured_type): Rename to...
	(dwarf2_per_bfd::allocate_signatured_type): ... this.
	(get_gdb_index_contents_ftype): Change parameter from
	dwarf2_per_objfile to dwarf2_per_bfd.
	* dwarf2/macro.c, dwarf2/index-write.c: Replace references to
	dwarf2_per_objfile with dwarf2_per_objfile->per_bfd.

Change-Id: I7de7b5d1ce7494aa73bfcf15f719d3c5c46e138c