CSharp: Add 'debug:portable' to v142 flag table for VS

Templates/MSBuild/FlagTables/v142_CSharp.json

@@ -200,6 +200,13 @@
     "value": "pdbonly",
     "flags": []
   },
+  {
+    "name":  "DebugType",
+    "switch": "debug:portable",
+    "comment": "",
+    "value": "portable",
+    "flags": []
+  },
   {
     "name":  "Optimize",
     "switch": "optimize",