Fix clang compiler errors and warnings in DQMServices/Core

[Dr15Jones]

clang was failing to compile becuase of unused variable errors in
DQMStore. Also fixed warnings about forward declarations not being
the same type as struct/class declarations.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_2_X.

Fix clang compiler errors and warnings in DQMServices/Core

It involves the following packages:

DQMServices/Core

@ojeda, @danduggan, @rovere, @cmsbuild, @nclopezo, @deguio, @Degano can you please review it and eventually sign? Thanks.
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[Dr15Jones]

This fixes the errors found in the CMSSW_7_2_CLANG_X_2014-07-01-0200 IB:
https://cmssdt.cern.ch/SDT/cgi-bin/buildlogs/slc6_amd64_gcc481/CMSSW_7_2_CLANG_X_2014-07-01-0200/DQMServices/Core

@ktf clang is also warning about unknown options which must be coming from scram:
warning: unknown warning option '-Werror=return-local-addr'; did you mean '-Werror=return-stack-address'? [-Wunknown-warning-option]

[cmsbuild]

+1
Tested at: 0660564
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-4475/235/summary.html

[deguio]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_2_X IBs unless changes (tests are also fine).

[ktf]

I also fixed the issue with the flag in cms-sw/cmsdist#829