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
- evaluate 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 evaluate 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.

To avoid storing the gdbarch information in the evaluator object, that
information is now always acquired from the per_objfile object.

All data members are now private and only visible to the evaluator
class, so a m_ prefix was added to all of their names to reflect that.
To make this distinction clear, they are also accessed through objects
this pointer, wherever that was not the case before.

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::evaluate): New function.
	(dwarf_expr_context::execute_stack_op): Change to use
	property_addr_info structure.
	* dwarf2/expr.h (struct dwarf_expr_context): New evaluate
	declaration. Change eval and fetch_result method to private.
        (dwarf_expr_context::gdbarch): Remove member.
        (dwarf_expr_context::stack): Make private and add m_ prefix.
        (dwarf_expr_context::addr_size): Make private and add
        m_ prefix.
        (dwarf_expr_context::recursion_depth): Make private and add
        m_ prefix.
        (dwarf_expr_context::max_recursion_depth): Make private and
        add m_ prefix.
        (dwarf_expr_context::len): Make private and add m_ prefix.
        (dwarf_expr_context::data): Make private and add m_ prefix.
        (dwarf_expr_context::initialized): Make private and add
        m_ prefix.
        (dwarf_expr_context::pieces): Make private and add m_ prefix.
        (dwarf_expr_context::per_objfile): Make private and add
        m_ prefix.
        (dwarf_expr_context::frame): Make private and add m_ prefix.
        (dwarf_expr_context::per_cu): Make private and add m_ prefix.
        (dwarf_expr_context::addr_info): Make private and add
        m_ prefix.
	* dwarf2/frame.c (execute_stack_op): Change to call evaluate
	method.
	* dwarf2/loc.c (dwarf2_evaluate_loc_desc_full): Change to call
	evaluate method.
	(dwarf2_locexpr_baton_eval): Change to call evaluate method.