generator: fix absname for fields with only accessor function

generator: fix absname for fields with only accessor function

Fix _c_helper_absolute_name for fields which cannot be accessed
as a struct/union member but which can be accessed by an
accessor function.

The fix calls the accessor function in these cases.

Example:
<struct name="AbsnameTest">
   <field type="CARD32" name="len" />
   <list type="CARD8" name="mylist1">
       <fieldref>len</fieldref>
   </list>
   <list type="CARD8" name="mylist2">
       <sumof ref="mylist1"/>
   </list>
</struct>

The sumof-expression ( <sumof ref="mylist1"/> ) refers to mylist1
which is only acessible by an accessor function.

Previously, sumof was only used inside bitcases,
where such lists are accessible by members of the
deserialized parent struct.
(there is a difference between deserialization of switches
and structs.)

V2 of this patch:
* replaced "!= None" with "is not None" because that's more pythonic.
  (according to suggestion from Ran Benita)

Message-ID: <5444E17D.3040206@DemoRecorder.com>
Patch-Thread-Subject: [Xcb] xinput: make ListInputDevices work, sumof with nested expr, ...
Patch-Set: ListInputDevices
Patch-Number: libxcb 1/6
Patch-Version: V2
Signed-off-by: Christian Linhart <chris@DemoRecorder.com>