[Attributor] Ensure AAIsDead correctly overrides getIRPosition

[RKSimon]

Summary

As commented on D65712, the getIRPosition methods weren't correctly being overridden.

[jdoerfert]

LGTM.

include/llvm/Transforms/IPO/Attributor.h

template <typename T> bool isLiveInstSet(T begin, T end) const {
     }
 
     return false;
   }
 
   /// Return an IR position, see struct IRPosition.
   ///
   ///{
-  IRPosition &getIRPosition() { return *this; }
-  const IRPosition &getIRPosition() const { return *this; }
+  IRPosition &getIRPosition() override { return *this; }
+  const IRPosition &getIRPosition() const override { return *this; }
   ///}
 
   /// Create an abstract attribute view for the position \p IRP.
   static AAIsDead &createForPosition(const IRPosition &IRP, Attributor &A);
 
   /// Unique ID (due to the unique address)
   static const char ID;
 };