[mlir][spirv] Use SmallVector<ArrayRef> for availability queries

Previously extensions and capabilities requirements are returned as
SmallVector<SmallVector>. It's an anti-pattern; this commit improves
a bit by returning as SmallVector<ArrayRef>. This is possible because
the internal sequence is always known statically (from the spec)
so that we can use a static constant array for it and get an ArrayRef.

Differential Revision: https://reviews.llvm.org/D75874