`getDescriptorFromTensorTuple` and `getMutDescriptorFromTensorTuple`
extract the tensor type from an `unrealized_conversion_cast` op that
serves as a workaround for missing 1:N dialect conversion support.
This commit changes these functions so that they explicitly receive the
tensor type as a function argument. This is in preparation of merging
the 1:1 and 1:N conversion drivers. The conversion patterns in this file
will soon start receiving multiple SSA values (`ValueRange`) from their
adaptors (instead of a single value that is the result of
`unrealized_conversion_cast`). It will no longer be possible to take the
tensor type from the `unrealized_conversion_cast` op. The
`unrealized_conversion_cast` workaround will disappear entirely.
Separates actual transformation files from supporting utility files in
the transforms directory. Includes a bazel overlay fix for the build (as
well as a bit of cleanup of that file to be less verbose and more
flexible).
