RemoteInspection: Handle ObjC typerefs in computeUnalignedFieldStartOffset (#76678)

This change is part of a fix for a regression that wasn't noticed because lldb has a 
fallback to using ASTContexts when type metadata resolution fails.

For any Swift class that directly or indirectly inherits from an ObjC class (such as 
`NSObject`, `NSView`, etc), the function `computeUnalignedFieldStartOffset` will fail 
to compute the start offset. To compute the start offset, it traverses the parent 
classes and uses their sizes. Once the traversal reaches an ObjC class, the 
RemoteInspection does not know the size. The fix here is to get the size from the 
`TypeInfo` returned by the external `TypeInfoProvider` (which in LLDB is 
`LLDBTypeInforProvider`).

Depended on by swiftlang/llvm-project#9320

Author: kastiglione

include/swift/RemoteInspection/ReflectionContext.h

@@ -1287,6 +1287,12 @@ class ReflectionContext
       // after the isa and retain fields.
       return isaAndRetainCountSize;
 
+    // `ObjCClassTypeRef` instances represent classes in the ObjC module ("__C").
+    // These will never have Swift type metadata.
+    if (auto *objcSuper = dyn_cast<ObjCClassTypeRef>(superclass))
+      if (auto *superTI = ExternalTypeInfo->getTypeInfo(objcSuper->getName()))
+        return superTI->getSize();
+
     auto superclassStart =
         computeUnalignedFieldStartOffset(superclass, ExternalTypeInfo);
     if (!superclassStart)