Empty base optimization, sharing vptrs

[Jason Merrill]

>>>>> Matt Austern <austern at isolde.engr.sgi.com> writes:

 > Another goof I just noticed: if it's a virtual but nonpolymorphic
 > base class then we can't put it at offset 0, because the derived
 > class has a vptr at offset 0 and the base class doesn't.

If it's empty and nonpolymorphic, then it doesn't matter where we put it
because its address will never be used for anything.

Jason