[clang] [llvm] [AArch64] Implement GCS ACLE intrinsics (PR #96903)

[John Brawn via llvm-commits]

john-brawn-arm wrote:

I've added a new change to work around a problem in intrinsic handling that's detailed in https://discourse.llvm.org/t/intrinsic-with-sideeffect-is-optimized-out/66053 (I previously had this workaround in, but removed it because it looked like it was no longer needed, but actually I just hadn't tested it enough).

https://github.com/llvm/llvm-project/pull/96903