Add resolve_dynamic_field

Add resolve_dynamic_field

The final patch in this series will change one dynamic property
approach to use a struct field rather than an offset and a field type.
This is convenient because the reference in DWARF is indeed to a field
-- and this approach lets us reuse the field-extraction logic that
already exists in gdb.

However, the field in question may have dynamic properties which must
be resolved before it can be used.  This patch prepares for this by
introducing a separate resolve_dynamic_field function.

This patch should cause no visible changes to behavior.