[clang] Reject incomplete types in `__is_layout_compatible()` (#87869)

This is a follow-up to #81506. As discussed in #87737, we're rejecting
incomplete types, save for exceptions listed in the C++ standard (`void`
and arrays of unknown bound). Note that arrays of unknown bound of
incomplete types are accepted.

Since we're happy with the current behavior of this intrinsic for
flexible array members
(https://github.com/llvm/llvm-project/pull/87737#discussion_r1553652570),
I added a couple of tests for that as well.