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)

include/swift/SIL/SILInstruction.h

@@ -8679,6 +8679,11 @@ class DestroyValueInst
   }
 
 public:
+  /// True if this destroy fully deinitializes the type by invoking the
+  /// user-defined deinitializer if present. This returns false if a prior
+  /// drop_deinit is present.
+  bool isFullDeinitialization();
+
   /// If true, then all references within the destroyed value will be
   /// overwritten with a sentinel. This is used in debug builds when shortening
   /// non-trivial value lifetimes to ensure the debugger cannot inspect invalid
@@ -8749,11 +8754,12 @@ public:
 /// for details. See SILVerifier.cpp for constraints on valid uses.
 class DropDeinitInst
     : public UnaryInstructionBase<SILInstructionKind::DropDeinitInst,
-                                  SingleValueInstruction> {
+                                  OwnershipForwardingSingleValueInstruction> {
   friend class SILBuilder;
 
   DropDeinitInst(SILDebugLocation DebugLoc, SILValue operand)
-      : UnaryInstructionBase(DebugLoc, operand, operand->getType()) {}
+    : UnaryInstructionBase(DebugLoc, operand, operand->getType(),
+                           OwnershipKind::Owned) {}
 };
 
 /// Equivalent to a copy_addr to [init] except that it is used for diagnostics
@@ -11134,6 +11140,7 @@ OwnershipForwardingSingleValueInstruction::classof(SILInstructionKind kind) {
   case SILInstructionKind::ThinToThickFunctionInst:
   case SILInstructionKind::UnconditionalCheckedCastInst:
   case SILInstructionKind::FunctionExtractIsolationInst:
+  case SILInstructionKind::DropDeinitInst:
     return true;
   default:
     return false;