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

[John McCall via cfe-commits]

rjmccall wrote:

> Alright. I think the LTO should be done as a follow-up PR since it will likely take some time and multiple reviews to get right.

Yes, that's a fine plan.  I expect that that'll be a significant amount of work.

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