[Clang][AArch64][SME] Add intrinsics for reading streaming vector length

This patch adds support for the following SME ACLE intrinsics (as defined
in https://arm-software.github.io/acle/main/acle.html

):

  - svcntsb
  - svcntsh
  - svcntsw
  - svcntsd

Co-authored-by: Sagar Kulkarni <sagar.kulkarni1@huawei.com>

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D134679