VS: Update v141 CL flag table for VS 15.9

Run the command

    python3 Source/cmConvertMSBuildXMLToJSON.py -t v141 \
        'c:/.../Common7/IDE/VC/VCTargets/1033/cl.xml'

To generate `v141_CL.json` locally.  Manually move new and updated
settings over to `Templates/MSBuild/FlagTables/v141_CL.json`.