[clang] [llvm] [mlir] [mlir][OpenMP] Convert omp.cancel sections to LLVMIR (PR #137193)

[Tom Eccles via cfe-commits]

tblah wrote:

It turned out that the Windows failures were due to some UB in my patch. Clang (with the OMPIRBuilder backend) was actually doing almost exactly the same hack as I implement in this patch, which then interacted badly by my attempts to do the same thing in OMPIRBuilder.

I have removed the clang code because I think it is better to have the hack in one place in OMPIRBuilder instead of in two different callbacks.

https://github.com/llvm/llvm-project/pull/137193