[cxx-interop] Skip type metadata for C++ types that are only used in private C++ fields

This fixes compiler errors for C++ types that use pimpl idiom:
```
invalid application of 'sizeof' to an incomplete type
```

rdar://141960396

Author: egorzhdan

lib/IRGen/GenReflection.cpp

@@ -931,6 +931,15 @@ class FieldTypeMetadataBuilder : public ReflectionMetadataBuilder {
 
     B.addInt32(getNumFields(NTD));
     forEachField(IGM, NTD, [&](Field field) {
+      // Skip private C++ fields that were imported as private Swift fields.
+      // The type of a private field might not have all the type witness
+      // operations that Swift requires, for instance,
+      // `std::unique_ptr<IncompleteType>` would not have a destructor.
+      if (field.getKind() == Field::Kind::Var &&
+          field.getVarDecl()->getClangDecl() &&
+          field.getVarDecl()->getFormalAccess() == AccessLevel::Private)
+        return;
+
       addField(field);
     });
   }