Add HepMCProduct backward compatibility for premixing

[wmtan]

Recent PR #10858 implemented a fix to avoid updating HepMCProduct in place. Backward compatibility was implemented by updating file Configuration/StandardSequences/python/Digi_cff.py.
However, workflows using premixed pileup with the data mixing module do not use Digi_cff.py.
They use DigiDMPreMix_cff.py instead. This PR implements backward compatibility in that file, thereby fixing the 14 DataMixer relvals that now fail in recycling mode.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_7_6_X.

Add HepMCProduct backward compatibility for premixing

It involves the following packages:

Configuration/StandardSequences

@cmsbuild, @franzoni, @davidlange6 can you please review it and eventually sign? Thanks.
@ghellwig, @makortel, @GiacomoSguazzoni, @rovere, @VinInn, @Martin-Grunewald, @cerati, @dgulhan this is something you requested to watch as well.
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@Degano you are the release manager for this.
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[wmtan]

@cmsbuild please test

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

+1
Tested at: e454140
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-11639/8624/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-11639/8624/summary.html