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

[Florian Hahn via cfe-commits]

https://github.com/fhahn commented:

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

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