Add new computed struct value callback interface

Add new computed struct value callback interface

At this point all support is there to add a new callback interface
for the computed struct value infrastructure.

Original callback interface (piece closure) is going to be removed as
soon as the switch to the new DWARF entry classes is done in the next
few patches.

gdb/ChangeLog:

        * dwarf2/expr.c (class computed_closure): New class.
        (closure_value_funcs): New closure callback structure.
        (copy_value_closure): New function.
        (free_value_closure): New function.
        (rw_closure_value): New function.
        (check_synthetic_pointer): New function.
        (write_closure_value): New function.
        (read_closure_value): New function.
        (is_optimized_out_closure_value): New function.
        (indirect_closure_value): New function.
        (coerce_closure_ref): New function.