Workaround for virtual function/dllimport bug, also patch guidance request

[Jason Merrill]

>>>>> Mumit Khan <khan@nanotech.wisc.edu> writes:

 > The fix is simple -- just import the vtables from the DLL. Currently,
 > config/winnt.c:associated_type returns a NULL context for a vtable,
 > and I don't know how to get to it, so it's not imported correctly. 

That's odd; DECL_CONTEXT for the vtable should give the right answer.

 > Re check_override, I've moved it to grokclassfn, and it looks ok so far.
 > Note that I haven't removed it from finish_struct_1 since I'm not sure
 > where else it needs to be added, other than the current one in 
 > add_implicitly_declared_members.

Please try removing it from everywhere but grokclassfn and see how that
works.

Jason