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

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Sirraide wrote:

> Makes sense to me, but do we have tests that make sure codegen for matrixes with power-of-2 bitints is sane?

I’ll check and if not, I’ll add some

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