[mlir] add transform dialect entry point

Introduce `transform::applyTransforms` as a top-level entry point to the
Transform dialect-driven transformation infrastructure, by analogy with
`applyFull/PartialConversion`. Clients are expected to use this function
and no longer need to maintain the transformation state. Make the
constructor of the TransformState private for that purpose.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D135681