Adds basic support for emitting delayed privatizers from flang. So far,
only types of symbols are supported (i.e. scalars), support for more
complicated types will be added later. This also makes sure that
reduction and delayed privatization work properly together by merging
the
body-gen callbacks for both in case both clauses are present on the
parallel construct.
This started as an experiment to reduce the compilation time of
iterating over `Lower/OpenMP.cpp` a bit since it is too slow at the
moment. Trying to do that, I split the `DataSharingProcessor`,
`ReductionProcessor`, and `ClauseProcessor` into their own files and
extracted some shared code into a util file. All of these new `.h/.cpp`
files as well as `OpenMP.cpp` are now under a `Lower/OpenMP/` directory.
This resulted is a slightly better organization of the OpenMP lowering
code and hence opening this NFC.
As for the compilation time, this unfortunately does not affect it much
(it shaves off a few seconds of `OpenMP.cpp` compilation) since from
what I learned the bottleneck is in `DirectivesCommon.h` and
`PFTBuilder.h` which both consume a lot of time in template
instantiation it seems.
