[clang] [OpenACC][CIR] Implement atomic-write lowering (PR #164627)

[Erich Keane via cfe-commits]

erichkeane wrote:

I realize my use of `Expr` in this patch as a variable is problematic, I see the build bot failures.  I'm locally validating a patch to fix that now, and will merge/fix that ASAP.

https://github.com/llvm/llvm-project/pull/164627