Add indirect_implicit_ptr to dwarf_location class

Add indirect_implicit_ptr to dwarf_location class

Similarly to the is_implicit_ptr_at method, the existing function
callback interface of the computed struct value, requiers a way to
apply indirection to an implicit pointer on a given offset of a given
length of an underlying location description.

This is different than reading from a struct value object (previously
described write_to_gdb_value method) in a way that the result of this
operation is expected to be a struct value of a pointed source level
variable instead of reading the value of that variable.

In the same way this is also different operation than the deref method
because the deref returns a read value of a given type from that
location description.

gdb/ChangeLog:

        * dwarf2/expr.c (dwarf_location::indirect_implicit_ptr):
        New method.
        (dwarf_implicit_pointer::indirect_implicit_ptr): New method.
        (dwarf_composite::indirect_implicit_ptr): New method.