[Clang][Sema] Do not accept "vector _Complex" for AltiVec/ZVector (#90467)

The AltiVec (POWER) and ZVector (IBM Z) language extensions do not
support using the "vector" keyword when the element type is a complex
type, but current code does not verify this.

Add a Sema check and diagnostic for this case.

Fixes: https://github.com/llvm/llvm-project/issues/88399