[move-only] Fix dead code elimination to preserve value deinits.

(cherry picked from commit 1dcc240)

Author: atrick

include/swift/SIL/SILMoveOnlyDeinit.h

@@ -61,7 +61,10 @@ class SILMoveOnlyDeinit final : public SILAllocated<SILMoveOnlyDeinit> {
 
   NominalTypeDecl *getNominalDecl() const { return nominalDecl; }
 
-  SILFunction *getImplementation() const { return funcImpl; }
+  SILFunction *getImplementation() const {
+    assert(funcImpl);
+    return funcImpl;
+  }
 
   IsSerialized_t isSerialized() const {
     return serialized ? IsSerialized : IsNotSerialized;

lib/SILOptimizer/IPO/DeadFunctionElimination.cpp

@@ -71,6 +71,8 @@ class DeadFunctionAndGlobalElimination {
     }
 
     /// Adds an implementation of the method in a specific conformance.
+    ///
+    /// \p Conf is null for default implementations and move-only deinits
     void addWitnessFunction(SILFunction *F, RootProtocolConformance *Conf) {
       assert(isWitnessMethod);
       implementingFunctions.push_back(FuncImpl(F, Conf));
@@ -616,6 +618,14 @@ class DeadFunctionAndGlobalElimination {
       if (dw.getVJP())
         ensureAlive(dw.getVJP());
     }
+
+    // Collect move-only deinit methods.
+    //
+    // TODO: Similar to addWitnessFunction, track the associated
+    // struct/enum decl to allow DCE of unused deinits.
+    for (auto *deinit : Module->getMoveOnlyDeinits()) {
+      makeAlive(deinit->getImplementation());
+    }
   }
 
   /// Removes all dead methods from vtables and witness tables.