Simple heuristic for identifying purecall

[sei-eschwartz]

From https://github.com/NationalSecurityAgency/ghidra/blob/master/Ghidra/Features/Decompiler/ghidra_scripts/classrecovery/RTTIWindowsClassRecoverer.java#L1377

// use vftables with references to all the same function (except possibly one deleting
// destructor)to find the purecall function
identifyPureVirtualFunction(recoveredClasses);

@sei-ccohen have discussed a few mechanisms for identifying purecall in the past, but I think we somehow missed this pretty obvious idea.

It's a bit unclear if we would implement this in Prolog or C++.