Add write method to location description classes

Add write method to location description classes

After adding the interface for reading from the location, it also
makes sense to add the interface for writing.

To be clear, DWARF standard doesn't have a concept of writting to a
location, but because of the way how callback functions are used to
interact with the opaque implementation of the computed struct value
objects, the choice was to either use the existing DWARF entry classes
or to invent another way of representing the complexity behind those
computed objects.

Adding a write method seems to be a simpler option of the two.

gdb/ChangeLog:

        * dwarf2/expr.c (dwarf_location::write): New method.
        (dwarf_undefined::write): New method.
        (dwarf_memory::write): New method.
        (dwarf_register::write): New method.
        (dwarf_implicit::write): New method.
        (dwarf_implicit_pointer::write): New method.
        (dwarf_composite::write): New method.