[AutoDiff] Use LinkEntity::SecondaryPointer for diff witness (#82412)

If `LinkEntity::isTypeKind()` is true, `IRGenModule::getAddrOfLLVMVariable` assumes that we can safely call
`LinkEntity::getType()`, which does `reinterpret_cast` of `LinkEntity::Pointer` to `TypeBase *`. However, for SIL
differentiability witness, the pointer has `SILDifferentiabilityWitness *` type, which is not derived from `TypeBase`. So, such a cast is not allowed.

Just as with `ProtocolWitnessTableLazyAccessFunction` and `ProtocolWitnessTableLazyCacheVariable` link entity kinds (which are also type kinds), we should use `SecondaryPointer` instead of `Pointer` for storing payload here, while setting `Pointer` to `nullptr`.

Author: kovdan01

include/swift/IRGen/Linking.h

@@ -132,7 +132,7 @@ class LinkEntity {
   /// ValueDecl*, SILFunction*, or TypeBase*, depending on Kind.
   void *Pointer;
 
-  /// ProtocolConformance*, depending on Kind.
+  /// ProtocolConformance* or SILDifferentiabilityWitness*, depending on Kind.
   void *SecondaryPointer;
 
   /// A hand-rolled bitfield with the following layout:
@@ -772,8 +772,8 @@ class LinkEntity {
   void
   setForDifferentiabilityWitness(Kind kind,
                                  const SILDifferentiabilityWitness *witness) {
-    Pointer = const_cast<void *>(static_cast<const void *>(witness));
-    SecondaryPointer = nullptr;
+    Pointer = nullptr;
+    SecondaryPointer = const_cast<void *>(static_cast<const void *>(witness));
     Data = LINKENTITY_SET_FIELD(Kind, unsigned(kind));
   }
 
@@ -1684,7 +1684,7 @@ class LinkEntity {
 
   SILDifferentiabilityWitness *getSILDifferentiabilityWitness() const {
     assert(getKind() == Kind::DifferentiabilityWitness);
-    return reinterpret_cast<SILDifferentiabilityWitness *>(Pointer);
+    return reinterpret_cast<SILDifferentiabilityWitness *>(SecondaryPointer);
   }
 
   const RootProtocolConformance *getRootProtocolConformance() const {

lib/IRGen/IRGenModule.cpp

@@ -1445,7 +1445,7 @@ bool IRGenModule::IsWellKnownBuiltinOrStructralType(CanType T) const {
       T == Context.getAnyObjectType())
     return true;
 
-  if (auto IntTy = dyn_cast<BuiltinIntegerType>(T)) {
+  if (auto IntTy = dyn_cast_or_null<BuiltinIntegerType>(T)) {
     auto Width = IntTy->getWidth();
     if (Width.isPointerWidth())
       return true;