[mlir][Vector] First step for 0D vector type

There seems to be a consensus that we should allow 0D vectors:
https://llvm.discourse.group/t/should-we-have-0-d-vectors/3097

This commit is only the first step: it changes the verifier and the parser to
allow vectors like `vector<f32>` (but does not allow explicit 0 dimensions,
i.e., `vector<0xf32>` is not allowed).

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D114086