[mlir][SCF] Add parallel abstraction on tensors.

This revision adds `scf.foreach_thread` and other supporting abstractions
that allow connecting parallel abstractions and tensors.

Discussion is available [here](https://discourse.llvm.org/t/rfc-parallel-abstraction-for-tensors-and-buffers/62607).

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D126555