[clang] [ObjC] Check entire chain of superclasses to see if class layout can be statically known (PR #81335)

[John McCall via cfe-commits]

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

> @rjmccall Are these tests adequate?

No.  You need to test the behavior of ivar accesses in the subclass when there is an intermediate superclass that has ivars declared in all the ways I identified.  Your test has many tests for ivar accesses in immediate subclasses of `NSObject`, which is an optimization that we've already been doing for several years.  It only tests indirect subclasses in one configuration, which is when the intermediate superclass has all its ivars declared in the `@interface`.

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