Remove dwarf2_per_cu_data::objfile ()

Remove dwarf2_per_cu_data::objfile ()

Since dwarf2_per_cu_data objects are going to become
objfile-independent, the backlink from dwarf2_per_cu_data to one
particular objfile must be removed.  Instead, users of
dwarf2_per_cu_data that need an objfile must know from somewhere else in
the context of which objfile they are using this CU.

This also helps remove a dwarf2_per_cu_data::dwarf2_per_objfile
reference (from where the objfile was obtained).

Note that the dwarf2_per_cu_data::objfile method has a special case to
make sure to return the main objfile, if the objfile associated to the
dwarf2_per_cu_data is a separate debug objfile.  I don't really know if
this is necessary: I ignored that, and didn't see any regression when
testing with the various Dejagnu boards with separate debug info, so I
presume it wasn't needed.  If it turns out this was needed, then we can
have a helper method on the objfile type for that.

gdb/ChangeLog:

	* dwarf2/read.h (struct dwarf2_per_cu_data) <objfile>: Remove.
	* dwarf2/read.c (dwarf2_compute_name): Pass per_objfile down.
	(read_call_site_scope): Assign per_objfile.
	(dwarf2_per_cu_data::objfile): Remove.
	* gdbtypes.h (struct call_site) <per_objfile>: New member.
	* dwarf2/loc.h (dwarf2_evaluate_loc_desc): Add
	dwarf2_per_objfile parameter.
	* dwarf2/loc.c (dwarf2_evaluate_loc_desc_full): Add
	dwarf2_per_objfile parameter.
	(dwarf_expr_reg_to_entry_parameter): Add output
	dwarf2_per_objfile parameter.
	(locexpr_get_frame_base): Update.
	(class dwarf_evaluate_loc_desc) <get_tls_address>: Update.
	<push_dwarf_reg_entry_value>: Update.
	<call_site_to_target_addr>: Update.
	(dwarf_entry_parameter_to_value): Add dwarf2_per_objfile
	parameter.
	(value_of_dwarf_reg_entry): Update.
	(rw_pieced_value): Update.
	(indirect_synthetic_pointer): Update.
	(dwarf2_evaluate_property): Update.
	(dwarf2_loc_desc_get_symbol_read_needs): Add dwarf2_per_objfile
	parameter.
	(locexpr_read_variable): Update.
	(locexpr_get_symbol_read_needs): Update.
	(loclist_read_variable): Update.