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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

(For context, grep for PreserveVec3Type.)

Probably the code should be defensive and check for the allocation size of the vector type, but I don't think any targets define the alignment for non-power-of-2 vectors in a way that would cause issues.

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