[clang] [mlir] [OpenACC][CIR] Implement 'private' clause lowering. (PR #151360)

[Erich Keane via cfe-commits]

erichkeane wrote:

If I don't hear objections in the next few hours on this, I'll merge this and follow up with the patch to do 'init' properly.

That one ended up becoming somewhat large because it had a bit of work in Sema to do, so I'm keeping it separate to minimize the size of this one.

https://github.com/llvm/llvm-project/pull/151360