[MLIR][Flang][OpenMP] Make omp.simdloop into a loop wrapper (#87365)

This patch updates the definition of `omp.simdloop` to enforce the
restrictions of a wrapper operation. It has been renamed to `omp.simd`,
to better reflect the naming used in the spec. All uses of "simdloop" in
function names have been updated accordingly.

Some changes to Flang lowering and OpenMP to LLVM IR translation are
introduced to prevent the introduction of compilation/test failures. The
eventual long term solution might be different.