Fix a bunch of clang warnings.

- Using struct consistently for ExpressionBase.
- Correct guard name.
Merge pull request #4156 from ktf/fix-clang-warnings

Fix a bunch of clang warnings.
Merge pull request #4157 from cms-sw/CMSSW_7_2_X

Forward port CMSSW_7_2_X into CMSSW_7_2_GEANT10_X