Move piece_closure and its support to expr.c

Move piece_closure and its support to expr.c

Following 5 patches series is trying to clean up the interface of the
DWARF expression evaluator class (dwarf_expr_context).

After merging all expression evaluators into one class, the next
logical step is to make a clean user interface for that class. To do
that, we first need to address the issue of class users writing and
reading the internal data of the class directly.

Fixing the case of writing is simple, it makes sense for an evaluator
instance to be per architecture basis. Currently, the best separation
seems to be per object file, so having that data (dwarf2_per_objfile)
as a constructor argument makes sense. It also makes sense to get the
address size from that object file, but unfortunately that interface
does not exist at the moment.

Luckily, address size information is already available to the users
through other means. As a result, the address size also needs to be a
class constructor argument, at least until a better interface for
acquiring that information from an object file is implemented.

The rest of the user written data comes down to a context of an
evaluated expression (compilation unit context, frame context and
passed in buffer context) and a source type information that a result
of evaluating expression is representing. So, it makes sense for all of
these to be arguments of an evaluation method.

To address the problem of reading the dwarf_expr_context class
internal data, we first need to understand why it is implemented that
way?

This is actualy a question of which existing class can be used to
represent both values and a location descriptions and why it is not
used currently?

The answer is in a struct value class/structure, but the problem is
that before the evaluators were merged, only one evaluator had an
infrastructure to resolve composite and implicit pointer location
descriptions.

After the merge, we are now able to use the struct value to represent
any result of the expression evaluation. It also makes sense to move
all infrastructure for those location descriptions to the expr.c file
considering that that is the only place using that infrastructure.

What we are left with in the end is a clean public interface of the
dwarf_expr_context class containing:

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

The idea with this particular patch is to move piece_closure structure
and the interface that handles it (lval_funcs) to expr.c file.

While implicit pointer location descriptions are still not useful in
the CFI context (of the AMD's DWARF standard extensions), the composite
location descriptions are certainly necessary to describe a results of
specific compiler optimizations.

Considering that a piece_closure structure is used to represent both,
there was no benefit in splitting them.

gdb/ChangeLog:

	* dwarf2/expr.c (struct piece_closure): Add from loc.c.
	(allocate_piece_closure): Add from loc.c.
	(bits_to_bytes): Add from loc.c.
	(rw_pieced_value): Add from loc.c.
	(read_pieced_value): Add from loc.c.
	(write_pieced_value): Add from loc.c.
	(check_pieced_synthetic_pointer): Add from loc.c.
	(indirect_pieced_value): Add from loc.c.
	(coerce_pieced_ref): Add from loc.c.
	(copy_pieced_value_closure): Add from loc.c.
	(free_pieced_value_closure): Add from loc.c.
	(sect_variable_value): Add from loc.c.
	* dwarf2/loc.c (sect_variable_value): Move to expr.c.
	(struct piece_closure): Move to expr.c.
	(allocate_piece_closure): Move to expr.c.
	(bits_to_bytes): Move to expr.c.
	(rw_pieced_value): Move to expr.c.
	(read_pieced_value): Move to expr.c.
	(write_pieced_value): Move to expr.c.
	(check_pieced_synthetic_pointer): Move to expr.c.
	(indirect_pieced_value): Move to expr.c.
	(coerce_pieced_ref): Move to expr.c.
	(copy_pieced_value_closure): Move to expr.c.
	(free_pieced_value_closure): Move to expr.c.