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

[via cfe-commits]

AtariDreams wrote:

Wait, @rjmccall   // Assume that anything declared with a forward declaration and no
  // @interface subclasses NSObject.
  
  Any reason why analysis can assume that but not codegen? Just curious.

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