VS: Update v142 CL flag table for VS 17.0 Preview 1

Run the command

    python3 Source/cmConvertMSBuildXMLToJSON.py -t v142 \
        'c:/.../MSBuild/Microsoft/VC/v160/1033/cl.xml'

To generate `v142_CL.json` locally.  Manually move new and updated
settings over to `Templates/MSBuild/FlagTables/v142_CL.json`.

Templates/MSBuild/FlagTables/v142_CL.json

@@ -475,6 +475,13 @@
     "value": "stdcpp14",
     "flags": []
   },
+  {
+    "name": "LanguageStandard",
+    "switch": "std:c++20",
+    "comment": "ISO C++20 Standard",
+    "value": "stdcpp20",
+    "flags": []
+  },
   {
     "name": "LanguageStandard",
     "switch": "std:c++17",