[clang] [llvm] [mlir] [OpenMP] Migrate GPU Reductions CodeGen from Clang to OMPIRBuilder (PR #80343)

[Akash Banerjee via cfe-commits]

TIFitis wrote:

@alexey-bataev I've added the test clang/test/OpenMP/reduction_complex.c to this PR. I tried to add the checks based on the other similar tests in the directory, let me know if it needs any changes.

https://github.com/llvm/llvm-project/pull/80343