Previously, these would set their ArrayRef members to reference their
storage SmallVectors after a copy-on-write operation. This leads to a
use-after-free if the builder is copied and the original destroyed (as
the new builder would still reference the old SmallVector).

The VectorType::Builder also set the ArrayRef<bool> scalableDims member
to a temporary SmallVector when the provided scalableDims are empty.
This again lead to a use-after-free, and is unnecessary as
VectorType::get already handles being passed an empty scalableDims
array.

These bugs were in-part caught by UBSAN, see:
https://lab.llvm.org/buildbot/#/builders/5/builds/37355

Easy to mess up 🤦

This is a safer helper class that avoids the issues with the previous
manual approaches. It also cleans up the builder implementations.

+ Some minor tweaks