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

[Florian Hahn via cfe-commits]

fhahn wrote:

For reference, as per https://clang.llvm.org/docs/MatrixTypes.html#matrix-type, the only allowed element types for matrixes are

* an integer type (as in C23 6.2.5p22), but excluding enumerated types and bool
* the standard floating types float or double
* a half-precision floating point type, if one is supported on the target

so we should either reject all BitInts or update the spec

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