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

[John McCall via cfe-commits]

rjmccall wrote:

Changing a type trait is generally always an ABI problem, but yeah, if this hasn't been released yet, we're still free to fix basic bugs.

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