SIL: Make drop_deinit a ForwardingInstruction.

drop_deinit forwards ownership while effectively stripping the deinitializer. It is similar to a type cast.

Fixes rdar://125590074 ([NonescapableTypes] Nonescapable types
cannot have deinits)

lib/SIL/IR/SILInstruction.cpp

@@ -1885,6 +1885,10 @@ bool MarkDependenceInst::visitNonEscapingLifetimeEnds(
   return !noUsers;
 }
 
+bool DestroyValueInst::isFullDeinitialization() {
+  return !isa<DropDeinitInst>(lookThroughOwnershipInsts(getOperand()));
+}
+
 PartialApplyInst *
 DestroyValueInst::getNonescapingClosureAllocation() const {
   SILValue operand = getOperand();