RFA: New ipa-devirt PATCH for c++/58678 (devirt vs. KDE)

[Jason Merrill]

This patch fixes the latest 58678 testcase by avoiding speculative 
devirtualization to the destructor of an abstract class, which can never 
succeed: you can't create an object of an abstract class, so the pointer 
must point to an object of a derived class, and the derived class 
necessarily has its own destructor.  Other virtual member functions of 
an abstract class are OK for devirtualization: the destructor is the 
only virtual function that is always overridden in every class.

We could also detect an abstract class by searching through the vtable 
for __cxa_pure_virtual, but I figured it was easy enough for the front 
end to set a flag on the BINFO.

Tested x86_64-pc-linux-gnu.  OK for trunk?
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