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

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cor3ntin wrote:

My 0 cents:

- I think deprecating the extension make sense
- In the meantime I think the builtin exists to support the library so the patch make sense as is (otherwise we have to prove it is used for something else). 

My understanding is that "0-length arrays" are mostly an unfortunate attempt at a flexible-member-arrays with sharper edges, so their support in the context of c++ type trait doesn't really make sense.

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