Rename and update the piece_closure structure

Rename and update the piece_closure structure

Class that describes a computed_lval closure needs to be update to fit
better with the new dwarf_entry set of classes. This also means that a
pieced_value_funcs interface with that closure, needs to be renamed and
updated accordingly.

Considering that a closure is designed to describe a computed location
description, it makes sense to rename piece_closure to a
computed_closure.

gdb/ChangeLog:

	* dwarf2/expr.c (struct piece_closure): Change to
	computed_closure class.
	(allocate_piece_closure): Remove function.
	(rw_pieced_value): Rename to rw_closure_value and change to use
	computed_closure class.
	(read_pieced_value): Rename to read_closure_value and change to
	use computed_closure class.
	(write_pieced_value): Rename to write_closure_value and change
	to use computed_closure class.
	(check_pieced_synthetic_pointer): Rename to
	check_synthetic_pointer and change to use computed_closure
	class.
	(indirect_pieced_value): Rename to indirect_closure_value and
	change to use computed_closure class.
	(coerce_pieced_ref): Rename to coerce_closure_ref and change
	to use computed_closure class.
	(copy_pieced_value_closure): Rename to copy_value_closure and
	change to use computed_closure class.
	(free_pieced_value_closure): Rename to free_value_closure and
	change to use computed_closure class.
	(dwarf_expr_context::gdb_value_to_dwarf_entry): Change to use
	computed_closure class.
	(dwarf_expr_context::dwarf_entry_to_gdb_value): Change to use
	computed_closure class.

Change-Id: Ice1df6e01c852bb8a94ba74e683d66fe57838036