[clang] [Clang] Fix __is_array returning true for zero-sized arrays (PR #86652)

[Jonathan Wakely via cfe-commits]

jwakely wrote:

> We don't use the trait currently in libc++ because of this bug and GCC only added it in trunk (and have the same bug), so probably not.

N.B. MSVC used to have the same bug as well, and it affected their `std::is_array` because MSVC auto-replaces `std::is_array<T>::value` with the intrinsic (and never instantiates the class template). I noted this behaviour in Feb 2022 and they fixed the intrinsic.

https://github.com/llvm/llvm-project/pull/86652