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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> Gate this on FEAT_PCDPHINT so that availability matches hardware support for the STSHH instruction. 

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.)

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