[mlir] Transform dialect: add named sequences

Named sequences introduce an additional abstraction and reuse capability
to the transform dialect. They can be though of as macros parameterized
with handles that can be invoked in places where a transform dialect
operation is expected. Such reuse was previously not possible in the
dialect and required dynamic construction of the transform IR from the
client language. Named sequences are intentionally restricted to
disallow recursion, as it could make the dialect accidentally
Turing-complete, which isn't desired at this point.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D146433