[clang] Reject VLAs in `__is_layout_compatible()` (#87737)

This is a follow-up to #81506. Since `__is_layout_compatible()` is a C++
intrinsic
(https://github.com/llvm/llvm-project/blob/ff1e72d68d1224271801ff5192a8c14fbd3be83b/clang/include/clang/Basic/TokenKinds.def#L523),
I don't think we should define how it interacts with VLA extension
unless we have a compelling reason to.

Since #81506 was merged after 18 cut-off, we don't have to follow any
kind of deprecation process.