Intoduce unchecked_ownership instruction in raw SIL

This instruction can be used to disable ownership verification on it's result and
will be allowed only in raw SIL.

Sometimes SILGen can produce invalid ownership SSA, that cannot be resolved until
mandatory passes run. We have a few ways to piecewise disable verification.
With unchecked_ownership instruction we can provide a uniform way to disable ownership
verification for a value.

lib/Serialization/SerializeSIL.cpp

@@ -1806,6 +1806,9 @@ void SILSerializer::writeSILInstruction(const SILInstruction &SI) {
     writeOneOperandExtraAttributeLayout(SI.getKind(), Attr, SI.getOperand(0));
     break;
   }
+  case SILInstructionKind::UncheckedOwnershipInst: {
+    llvm_unreachable("Invalid unchecked_ownership during serialzation");
+  }
   case SILInstructionKind::YieldInst: {
     auto YI = cast<YieldInst>(&SI);
     SmallVector<ValueID, 4> args;