[MLIR][Flang][OpenMP] Make omp.simdloop into a loop wrapper

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.