[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`.

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;