[clang] [llvm] [AArch64][clang][llvm] Add ACLE `stshh` atomic store builtin (PR #181386)

[Jonathan Thackray via cfe-commits]

jthackray wrote:

> Is this really necessary? STSHH has exactly the same semantics whether or not the target CPU supports FEAT_PCDPHINT. (It's a hint which the CPU is allowed to ignore.)

@efriedma-quic Thanks, good spot, I forgot it was a hint. Now fixed.

https://github.com/llvm/llvm-project/pull/181386