[flang] [llvm] [mlir] [flang][llvm][OpenMP][OpenACC] Add implicit casts to omp.atomic and acc.atomic (PR #131603)

[Tom Eccles via llvm-commits]

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

LGTM. Thank you for sticking with this patch.

@clementval @razvanlupusoru are you okay with the TODO affecting OpenACC? (please wait for an answer before merging)

https://github.com/llvm/llvm-project/pull/131603