[sil] Add a helper on ApplySite to set the ApplySite's SILValue callee in a generic way.

Just so that people do not have to touch operand numbers and potentially make a
mistake.

include/swift/SIL/ApplySite.h

@@ -297,6 +297,11 @@ public:
     getArgumentOperands()[i].set(V);
   }
 
+  void setCallee(SILValue V) const {
+    unsigned calleeIndex = getCalleeOperand()->getOperandNumber();
+    getInstruction()->getAllOperands()[calleeIndex].set(V);
+  }
+
   /// Return the operand index of the first applied argument.
   unsigned getOperandIndexOfFirstArgument() const {
     FOREACH_IMPL_RETURN(getArgumentOperandNumber());