Move frame context info to dwarf_expr_context

Move frame context info to dwarf_expr_context

Following 15 patches in this patch series is cleaning up the design of
the DWARF expression evaluator (dwarf_expr_context) to make future
extensions of that evaluator easier and cleaner to implement.

There are three subclasses of the dwarf_expr_context class
(dwarf_expr_executor, dwarf_evaluate_loc_desc and
evaluate_for_locexpr_baton). Here is a short description of each class:

- dwarf_expr_executor is evaluating a DWARF expression in a context
  of a Call Frame Information. The overridden methods of this subclass
  report an error if a specific DWARF operation, represented by that
  method, is not allowed in a CFI context. The source code of this
  subclass lacks the support for composite as well as implicit pointer
  location description.

- dwarf_evaluate_loc_desc can evaluate any expression with no
  restrictions. All of the methods that this subclass overrides are
  actually doing what they are intended to do. This subclass contains
  a full support for all location description types.

- evaluate_for_locexpr_baton subclass is a specialization of the
  dwarf_evaluate_loc_desc subclass and it's function is to add
  support for passed in buffers. This seems to be a way to go around
  the fact that DWARF standard lacks a bit offset support for memory
  location descriptions as well as using any location description for
  the push object address functionality.

It all comes down to this question: what is a function of a DWARF
expression evaluator?

Is it to evaluate the expression in a given context or to check the
correctness of that expression in that context?

Currently, the only reason why there is a dwarf_expr_executor subclass
is to report an invalid DWARF expression in a context of a CFI, but is
that what the evaluator is supposed to do considering that the evaluator
is not tied to a given DWARF version?

There are more and more vendor and GNU extensions that are not part of
the DWARF standard, so is it that impossible to expect that some of the
extensions could actually lift the previously imposed restrictions of
the CFI context? Not to mention that every new DWARF version is lifting
some restrictions anyway.

The thing that makes more sense for an evaluator to do, is to take the
context of an evaluation and checks the requirements of every operation
evaluated against that context. With this approach, the evaluator would
report an error only if parts of the context, necessary for the
evaluation, are missing.

If this approach is taken, then the unification of the
dwarf_evaluate_loc_desc, dwarf_expr_executor and dwarf_expr_context
is the next logical step. This makes a design of the DWARF expression
evaluator cleaner and allows more flexibility when supporting future
vendor and GNU extensions.

Additional benefit here is that now all evaluators have access to all
location description types, which means that a vendor extended CFI
rules could support composite location description as well. This also
means that a new evaluator interface can be changed to return a single
struct value (that describes the result of the evaluation) instead of
a caller poking around the dwarf_expr_context internal data for answers
(like it is done currently).

This patch starts the merging process by moving the frame context
information and support from dwarf_expr_executor and
dwarf_evaluate_loc_desc to dwarf_expr_context evaluator. The idea
is to report an error when a given operation requires a frame
information to be resolved, if that information is not present.

gdb/ChangeLog:

	* dwarf2/expr.c (ensure_have_frame): New function.
	(read_addr_from_reg): Add from frame.c.
	(dwarf_expr_context::dwarf_expr_context): Add frame info to
	dwarf_expr_context.
	(dwarf_expr_context::read_addr_from_reg): Remove.
	(dwarf_expr_context::get_reg_value): Move from
	dwarf_evaluate_loc_desc.
	(dwarf_expr_context::get_frame_base): Move from
	dwarf_evaluate_loc_desc.
	(dwarf_expr_context::execute_stack_op): Call frame context info
	check. Remove use of read_addr_from_reg method.
	* dwarf2/expr.h (struct dwarf_expr_context): Add frame info
	member, read_addr_from_reg, get_reg_value and get_frame_base
	declaration.
	(read_addr_from_reg): Move to expr.c.
	* dwarf2/frame.c (read_addr_from_reg): Move to
	dwarf_expr_context.
	(dwarf_expr_executor::read_addr_from_reg): Remove.
	(dwarf_expr_executor::get_frame_base): Remove.
	(dwarf_expr_executor::get_reg_value): Remove.
	(execute_stack_op): Use read_addr_from_reg function instead of
	read_addr_from_reg method.
	* dwarf2/loc.c (dwarf_evaluate_loc_desc::get_frame_base): Move
	to dwarf_expr_context.
	(dwarf_evaluate_loc_desc::get_reg_value): Move to
	dwarf_expr_context.
	(dwarf_evaluate_loc_desc::read_addr_from_reg): Remove.
	(dwarf2_locexpr_baton_eval):Use read_addr_from_reg function
	instead of read_addr_from_reg method.