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

[Mariya Podchishchaeva via cfe-commits]

https://github.com/Fznamznon commented:

It feels like that the non-power-of-2 _BitInt vectors were disabled because it wasn't known how to handle them. https://reviews.llvm.org/D133634#3793653
It probably is still so, however, does it make sense to document the new behavior? 
Hopefully also, since this never worked properly, this is not a breaking change.

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