Move get_object_address to dwarf_expr_context

Move get_object_address to dwarf_expr_context

Following the idea of merging the evaluators, the get_object_address
and can be moved from dwarf_expr_executor and dwarf_evaluate_loc_desc
classes to their base class dwarf_expr_context.

gdb/ChangeLog:

	* dwarf2/expr.c (dwarf_expr_context::get_object_address): Move
	from dwarf_evaluate_loc_desc.
	(class dwarf_expr_context): Add object address member to
	dwarf_expr_context.
	* dwarf2/expr.h (dwarf_expr_context::get_frame_pc): Remove
	method.
	* dwarf2/frame.c (dwarf_expr_executor::get_object_address):
	Remove method.
	* dwarf2/loc.c (dwarf_evaluate_loc_desc::get_object_address):
	move to dwarf_expr_context.
	(class dwarf_evaluate_loc_desc): Move object address member to
	dwarf_expr_context.