VS: Populate `/Y-` flag table entry for v142

Apply the change from commit 8df25f94 (VS: connect /Y- compiler option
with "Not Using Precompiled Headers", 2018-11-26) to the v142 flag
table.

Templates/MSBuild/FlagTables/v142_CL.json

@@ -478,7 +478,7 @@
   },
   {
     "name": "PrecompiledHeader",
-    "switch": "",
+    "switch": "Y-",
     "comment": "Not Using Precompiled Headers",
     "value": "NotUsing",
     "flags": []