[clang] [llvm] [AArch64] Implement the atomic store with hint intrinsic (PR #198316)

[via llvm-commits]

Lukacma wrote:

@efriedma-quic  Hint being droppable is actual desired feature if you look at the spec:

> The hint is a suggestion to the compiler and maps directly to a
specific hint instruction variant in the ISA. The compiler may use this hint
when selecting code sequences, but it is not required to emit a specific
hint instruction or a specific instruction sequence.

The idea is that we don't want to prevent compiler optimizations, just because user wants hint added as the benefit from optimizations is likely to be larger than from using the hint. That's why I think we still want to use store atomic as it probably has better compiler support.

https://github.com/llvm/llvm-project/pull/198316