Introduce a new null_class SIL instruction for forming a null pointer

reference to something of class type.  This is required to model
RebindSelfInConstructorExpr correctly to DI, since in the class case, 
self.init and super.init *take* a value out of class box so that it 
can pass the +1 value without performing an extra retain.  Nothing
else in the compiler uninitializes a DI-controlled memory object
like this, so nothing else needs this.  DI really doesn't like something
going from initialized to uninitialized.

Yes, I feel super-gross about this and am really unhappy about it.  I
may end up reverting this if I can find an alternate solution to this
problem.



Swift SVN r27525

lib/SILAnalysis/ValueTracking.cpp

@@ -92,6 +92,7 @@ static bool isTransitiveEscapeInst(SILInstruction *Inst) {
   case ValueKind::DeinitExistentialAddrInst:
   case ValueKind::UnreachableInst:
   case ValueKind::IsNonnullInst:
+  case ValueKind::NullClassInst:
   case ValueKind::CondFailInst:
   case ValueKind::DynamicMethodBranchInst:
   case ValueKind::ReturnInst: