C++ ABI: Proposed key function tweak

[Jason Merrill]

Since a pure virtual destructor must still be defined, we can use it as the
key function even though it's pure.  Making this change will allow users to
avoid unnecessary duplicate vtables for abstract classes.

Jason

*** abi.html.~1~	Sat Nov 18 00:13:27 2000
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*************** In either case, it must be weak.
*** 4715,4721 ****
  <p>
  The virtual table for a class is emitted in the same object containing
  the definition of its <i>key function</i>,
! i.e. the first non-pure virtual function
  that is not inline at the point of class definition.
  If there is no key function, it is emitted everywhere used.
  The emitted virtual table includes the full virtual table group for the class,
--- 4732,4738 ----
  <p>
  The virtual table for a class is emitted in the same object containing
  the definition of its <i>key function</i>,
! i.e. the first non-pure virtual function or pure virtual destructor
  that is not inline at the point of class definition.
  If there is no key function, it is emitted everywhere used.
  The emitted virtual table includes the full virtual table group for the class,