Make DWARF evaluator return a single struct value

Make DWARF evaluator return a single struct value

The patch is addressing the issue of class users writing and reading
the internal data of the dwarf_expr_context class.

At this point, all conditions are met for the DWARF evaluator to return
an evaluation result in a form of a single struct value object.

gdb/ChangeLog:

	* dwarf2/expr.c (pieced_value_funcs): Chenge to static
	function.
	(allocate_piece_closure): Change to static function.
	(dwarf_expr_context::fetch_result): New function.
	* dwarf2/expr.h (struct piece_closure): Remove declaration.
	(struct dwarf_expr_context): fetch_result new declaration.
	fetch, fetch_address and fetch_in_stack_memory members move
	to private.
	(allocate_piece_closure): Remove.
	* dwarf2/frame.c (execute_stack_op): Change to use
	fetch_result.
	* dwarf2/loc.c (dwarf2_evaluate_loc_desc_full): Change to use
	fetch_result.
	(dwarf2_locexpr_baton_eval): Change to use fetch_result.
        * dwarf2/loc.h (invalid_synthetic_pointer): Expose function.