[clang] [ObjC] Check entire chain of superclasses to see if class has fixed offsets (PR #81335)

[John McCall via cfe-commits]

https://github.com/rjmccall requested changes to this pull request.

Okay, your description of the change here is very misleading; it doesn't even mention that this is specific to having an `@implementation` for every intermediate class.

Your test seems completely inadequate — you need to test that we're doing the static layout properly, and you need to test with ivars coming from a variety of different places, including:
- ivars declared in the `@interface`
- ivars declared in the `@implementation`
- ivars synthesized explicitly in the `@implementation`
- vars synthesized implicitly in the `@implementation`

https://github.com/llvm/llvm-project/pull/81335