egcs-970907: covariant return type + virtual inheritance = core

[Jay Sachs]

> The attached program compiles & links fine, but dies with a bus error
> at the line marked (*1*).  Somewhat surprisingly, the value of the
> pointer printed just before that line is the same as in line (*2*).
> 
> There is no problem if class B inherits non-virtually from A.
> 
> This is on a Sparc Ultra, SunOS 5.5.1.

I should add that it doesn't matter whether it's compiled with or
without -frtti.

-jay