Bugfix #2514 main_v11.1 clang

[JohnHalleyGotway]

Expected Differences

Please see this comment for an explanation for why this change is needed.

• Do these changes introduce new tools, command line arguments, or configuration file options? [No]
  
  If yes, please describe:
  

• Do these changes modify the structure of existing or add new output data types (e.g. statistic line types or NetCDF variables)? [No]
  
  If yes, please describe:
  

Pull Request Testing

• Describe testing already performed for these changes:
  
  Confirmed that the bugfix branch compiles, but unfortunately, I did not test using clang.

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  
  Confirm that these changes are consistent with these changes in PR Bugfix #2514 develop clang #2563. Note that only concat_string.h needed to be fixed.

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes]
  No changes needed.

• Do these changes include sufficient testing updates? [No]
  Ideally we'd automate testing the compilation with clang via GitHub actions. But we don't have the time and resources to add that at this time.

• Will this PR result in changes to the test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

• Please complete this pull request review by [Thursday 7/27/23].
  

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[jprestop]

I have confirmed that these changes are consistent with these changes in PR #2563, noting that only concat_string.h needed to be fixed. I agree that ideally we'd automate testing the compilation with clang via GitHub actions, but we don't have the time and resources to add that at this time. Thanks for fixing this! I approve.