[SIL] Added on-stack pack metadata marker insts.

The new alloc_pack_metadata and dealloc_pack_metadata are inserted as
part of IRGen lowering.  The former indicates that the next instruction
might result in on-stack pack metadata being emitted.  The latter
indicates that this is the position at which metadata emitted on behalf
of its operand should be cleaned up.

lib/SILOptimizer/Utils/StackNesting.cpp

@@ -230,6 +230,9 @@ static SILInstruction *createDealloc(SILInstruction *Alloc,
                              SILType::getEmptyTupleType(context),
                              SubstitutionMap(), {bi});
     }
+    case SILInstructionKind::AllocPackMetadataInst:
+      return B.createDeallocPackMetadata(Location,
+                                         cast<AllocPackMetadataInst>(Alloc));
     default:
       llvm_unreachable("unknown stack allocation");
   }