[clang] [Clang] [Sema] Reject non-power-of-2 `_BitInt` matrix element types (PR #117487)

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Sirraide wrote:

I’ve added some tests and also updated the docs to mention that non-power-of-2 `_BitInt`s are not valid matrix element types.

https://github.com/llvm/llvm-project/pull/117487