[mlir][transform] change RaggedArray internals

Change the internal storage scheme from storing a MutableArrayRef to
storing an explicit offset+length pair. Storing an ArrayRef is dangerous
because it contains the pointer to the first element in the range, but
the entire storage vector may be reallocated, making the pointer
dangling. We don't know when the reallocation happends, so we can't
update the ArrayRefs. Store the explicit offset instead and construct
ArrayRefs on-the-fly.

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D146239