[ARM,MVE] Add intrinsics vclzq and vclsq.

Summary:
vclzq maps nicely to the existing target-independent @llvm.ctlz IR
intrinsic. But vclsq ('count leading sign bits') has no corresponding
target-independent intrinsic, so I've made up @llvm.arm.mve.vcls.

This commit adds the unpredicated forms only.

Reviewers: dmgreen, miyuki, MarkMurrayARM, ostannard

Reviewed By: miyuki

Subscribers: kristof.beyls, hiraditya, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D74335