[mlir] scf::ForOp: provide builders with callbacks for loop body

Thanks to a recent change that made `::build` functions take an instance of
`OpBuilder`, it is now possible to build operations within a region attached to
the operation about to be created. Exercise this on `scf::ForOp` by taking a
callback that populates the loop body while the loop is being created.

Additionally, provide helper functions to build perfect nests of `ForOp`s,
with support for iteration arguments. These functions provide the same
functionality as EDSC LoopNestBuilder with simpler implementation, without
relying on edsc::ScopedContext, and using `OpBuilder` in an unambiguous way.
Compatibility functions for EDSC are provided, but may be removed in the
future.

Differential Revision: https://reviews.llvm.org/D79688

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