[clang] [llvm] [Clang] Fix definition of layout-compatible to ignore empty classes (PR #92103)

[Mital Ashok via cfe-commits]

MitalAshok wrote:

I am also looking to cherry-pick this for Clang 19 so there won't be a difference between Clang 19 (when `__is_layout_compatible` was added) and Clang 20

https://github.com/llvm/llvm-project/pull/92103