Add to_value method to dwarf_location class

Add to_value method to dwarf_location class

Following the idea from the last patch this patch is adding a
conversion method from any dwarf_location derived object into
a dwarf_value object.

Currently, we only know how to convert from a memory location
description into a value, but it is resonable to expect a set
of target hooks that would let the target decide on how to do
other conversions in the future.

gdb/ChangeLog:

        * dwarf2/expr.c (dwarf_location::to_value): New method.
        (dwarf_memory::to_value): New method.
        (ill_formed_expression): New function.