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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

The type of an alloca is only used to determine the number of bytes allocated; it doesn't have any other semantics.  So it's fine as long as the alloca is 16 bytes.  Which I think it is in the given example, because of the datalayout rules for `<3 x i32>`.

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