[clang] [flang] [llvm] [mlir] [OpenMP][flang] Add initial support for by-ref reductions on the GPU (PR #165714)

[Tom Eccles via llvm-commits]

https://github.com/tblah approved this pull request.

LGTM so long as the people who understand GPUs are happy too :)

I would suggest restricting the new data pointer region to only a single block, but that's up to you.

https://github.com/llvm/llvm-project/pull/165714