[clang] [OpenACC][CIR] Implement 'reduction' combiner lowering for 5 ops (PR #162906)

[Erich Keane via cfe-commits]

erichkeane wrote:

Sema patch this depends on reverted here https://github.com/llvm/llvm-project/pull/162920

So we'll have to wait to merge this until that one gets re-added.

https://github.com/llvm/llvm-project/pull/162906