[flang][Lower][OpenMP][NFC] tidy up PrivateReductionUtils (#125867)

First part of a series of patches to improve private/reduction init and
cleanup region generation.

This commit is NFC. I factored out processing for each datatype into its
own method so that it is easier to keep track of what is being handled
where (I found the old gigantic init region generation function
difficult to navigate). The methods all share context in a helper class
to avoid having to pass a very large number of arguments.

I also removed the conflation between the mold argument and the mold
argument after loading. This should make it easier to avoid generating
dead uses of the mold argument in a later non-nfc patch.

Author: tblah