Remove empty frame and full evaluators

Remove empty frame and full evaluators

There are no virtual methods that require different specialization in
dwarf_expr_context class. This means that derived classes
dwarf_expr_executor and dwarf_evaluate_loc_desc are not needed any
more.

As a result of this, the  evaluate_for_locexpr_baton class base class
is now the dwarf_expr_context class.

There might be a need for a better class hierarchy when we know more
about the direction of the future DWARF versions and gdb extensions,
but that is out of the scope of this patch series.

gdb/ChangeLog:

	* dwarf2/frame.c (class dwarf_expr_executor): Remove class.
	(execute_stack_op): Instantiate dwarf_expr_context instead of
	dwarf_evaluate_loc_desc class.
	* dwarf2/loc.c (class dwarf_evaluate_loc_desc): Remove class.
	(dwarf2_evaluate_loc_desc_full): Instantiate dwarf_expr_context
	instead of dwarf_evaluate_loc_desc class.
	(struct evaluate_for_locexpr_baton): Derive from
	dwarf_expr_context.