[AArch64][SVE] Allocate locals that are scalable vectors.

This patch adds a target interface to set the StackID for a given type,
which allows scalable vectors (e.g. `<vscale x 16 x i8>`) to be assigned a
'sve-vec' StackID, so it is allocated in the SVE area of the stack frame.

Reviewers: ostannard, efriedma, rengolin, cameron.mcinally

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D70080