Change DWARF stack to use new dwarf_entry classes

Change DWARF stack to use new dwarf_entry classes

To replace existing DWARF stack element (dwarf_stack_value) with
dwarf_entry class based objects, a support for conversion between
struct value and the new classes is needed. The reason for this is
that dwarf_entry based classes are not designed to be visible outside
the expr.c file. This makes the DWARF expression evaluator more self
contained. This can be beneficial if there is ever a need to have a
DWARF support in gdbserver.

Once the conversion support is added, the current DWARF stack element
can easily be swapped out.

gdb/ChangeLog:

	* dwarf2/expr.c (dwarf_value_equal_op): New function.
	(dwarf_value_less_op): New function.
	(struct piece_closure): Change to use dwarf_entry based
	classes.
	(allocate_piece_closure): Change to use dwarf_entry based
	classes.
	(rw_pieced_value): Change to use dwarf_entry based classes.
	(check_pieced_synthetic_pointer): Change to use dwarf_entry
	based classes.
	(check_synthetic_pointer_location): New function.
	(indirect_pieced_value): Change to use dwarf_entry based
	classes.
	(indirect_from_location): New function.
	(coerce_pieced_ref): Change to use dwarf_entry based classes.
	(free_pieced_value_closure): Change to use dwarf_entry based
	classes.
	(dwarf_expr_context::~dwarf_expr_context): Instantiate
	dwarf_entry_factory object.
	(dwarf_expr_context::push): Change to use dwarf_entry based
	classes.
	(dwarf_expr_context::push_address): Change to use dwarf_entry
	based classes.
	(dwarf_expr_context::fetch): Change to use dwarf_entry based
	classes.
	(dwarf_expr_context::read_mem): Remove method.
	(dwarf_expr_context::fetch_result): Change to use dwarf_entry
	based classes.
	(dwarf_expr_context::dwarf_entry_deref): New method.
	(dwarf_expr_context::gdb_value_to_dwarf_entry): New method.
	(dwarf_expr_context::dwarf_entry_to_gdb_value): New method.
	(dwarf_expr_context::fetch_address): Change to use dwarf_entry
	based classes.
	(dwarf_expr_context::fetch_in_stack_memory): Change to use
	dwarf_entry based classes.
	(dwarf_expr_context::add_piece): Change to use dwarf_entry based
	classes.
	(dwarf_expr_context::execute_stack_op): Change to use dwarf_entry
	based classes.
	* dwarf2/expr.h (class dwarf_entry): New declaration.
	(class dwarf_entry_factory): New declaration.
	(enum dwarf_value_location): Remove enumeration.
	(struct dwarf_expr_piece): Remove structure.
	(struct dwarf_stack_value): Remove structure.
	(struct dwarf_expr_context): Change to use dwarf_entry based
	classes. Add dwarf_entry_factory object.

Change-Id: I828c9b087c8ab3f57d9b208b360bcf5688810586