Handle dynamic field properties

Handle dynamic field properties

I found a situation where gdb could not properly decode an Ada type.
In this first scenario, the discriminant of a type is a bit-field.
PROP_ADDR_OFFSET does not handle this situation, because it only
allows an offset -- not a bit-size.

My original approach to this just added a bit size as well, but after
some discussion with Eric Botcazou, we found another failing case: a
tagged type can have a second discriminant that appears at a variable
offset.

So, this patch changes this code to accept a general 'struct field'
instead of trying to replicate the field-finding machinery by itself.

This is handled at property-evaluation time by simply using a 'field'
and resolving its dynamic properties.  Then the usual field-extraction
function is called to get the value.

Because the baton now just holds a field, I renamed PROP_ADDR_OFFSET
to PROP_FIELD.

The DWARF reader now defers filling in the property baton until the
fields have been attached to the type.

Finally, I noticed that if the discriminant field has a biased
representation, then unpack_field_as_long would not handle this
either.  This bug is also fixed here, and the test case checks this.

Regression tested on x86-64 Fedora 41.