Simplify dwarf_expr_context class interface

Simplify dwarf_expr_context class interface

Idea of this patch is to get a clean and simple public interface for
the dwarf_expr_context class, looking like:

- constructor,
- destructor,
- push_address method and
- eval_exp method.

Where constructor should only ever require a target architecture
information. This information is held in per object file
(dwarf2_per_objfile) structure, so it makes sense to keep that
structure as a constructor argument. It also makes sense to get the
address size from that structure, but unfortunately that interface
doesn't exist at the moment, so the dwarf_expr_context class user
needs to provide that information.

The push_address method is used to push a CORE_ADDR as a value on
top of the DWARF stack before the evaluation. This method can be
later changed to push any struct value object on the stack.

The eval_exp method is the method that evaluates a DWARF expression
and provides the evaluation result, in a form of a single struct
value object that describes a location. To do this, the method requires
a context of the evaluation, as well as expected result type
information. If the type information is not provided, the DWARF generic
type will be used instead.

gdb/ChangeLog:

	* dwarf2/expr.c (dwarf_expr_context::dwarf_expr_context): Add
	address size argument.
	(dwarf_expr_context::read_mem): Change to use property_addr_info
	structure.
	(dwarf_expr_context::eval_exp): New function.
	(dwarf_expr_context::execute_stack_op): Change to use
	property_addr_info structure.
	* dwarf2/expr.h (struct dwarf_expr_context): New eval_exp
	declaration. Change eval and fetch_result method to private.
	* dwarf2/frame.c (execute_stack_op): Change to call eval_exp
	method.
	* dwarf2/loc.c (dwarf2_evaluate_loc_desc_full): Change to call
	eval_exp method.
	(dwarf2_locexpr_baton_eval): Change to call eval_exp method.

Change-Id: I8abd0dbf93e74a8a5adce143bfdea83c802233c3