Through a series of commits:

* f7fec28f (core: Refactor ADIOS factory to use Comm encapsulation, 2019-06-21, v2.5.0~66^2~35)
* 29177b01 (engine: Construct with Comm encapsulation, 2019-06-18, v2.5.0~66^2~31)
* 33715618 (helper: Factor out MPI-specific Comm interfaces, 2019-09-12)
* 89891238 (core: Replace MPI with Comm in ADIOS, IO, and Stream interfaces, 2019-10-07)

we converted the core::ADIOS constructor from taking a `MPI_Comm`
and calling `MPI_Comm_dup` internally to taking an instance of
`helper::Comm` by value, typically moved from an instance created
by `helper::CommFromMPI`.  Therefore `helper::CommFromMPI` calls
`MPI_Comm_dup` to preserve the existing semantics.

However, two commits introduced uses of `helper::CommFromMPI` in places
that did not previously use `MPI_Comm_dup`:

* 554a756f (utils: Port adios_reorganize to Comm encapsulation, 2019-09-19)
* 1ec1ec43 (sst: Re-implement SMPI_ interfaces as C wrappers of Comm encapsulation, 2020-02-11)

The calls updated by those commits are intend to create a `helper::Comm`
that uses `MPI_COMM_WORLD` without duplicating it.

In order to distinguish the cases that duplicate from those that do not,
replace `helper::CommFromMPI` with two variants:

* `helper::CommDupMPI` to duplicate the given `MPI_Comm`
* `helper::CommWithMPI` to take ownership of the given `MPI_Comm`

Update all the call sites to use the variant with appropriate semantics.

Fixes: #2008