VS: connect /Y- compiler option with "Not Using Precompiled Headers"

Original header commit v3.10.0-rc1~423^2

The change allows to selectively disable PrecompiledHeaders.
Despite the `$(VCTargetsPath)/1033/cl.xml` contains an empty value for switch,
for effectively turn off setting need to use /Y- option as described on msdn:
https://msdn.microsoft.com/en-us/library/1hy7a92h.aspx

Templates/MSBuild/FlagTables/v10_CL.json

@@ -366,7 +366,7 @@
   },
   {
     "name": "PrecompiledHeader",
-    "switch": "",
+    "switch": "Y-",
     "comment": "Not Using Precompiled Headers",
     "value": "NotUsing",
     "flags": []

Templates/MSBuild/FlagTables/v11_CL.json

@@ -387,7 +387,7 @@
   },
   {
     "name": "PrecompiledHeader",
-    "switch": "",
+    "switch": "Y-",
     "comment": "Not Using Precompiled Headers",
     "value": "NotUsing",
     "flags": []

Templates/MSBuild/FlagTables/v12_CL.json

@@ -415,7 +415,7 @@
   },
   {
     "name": "PrecompiledHeader",
-    "switch": "",
+    "switch": "Y-",
     "comment": "Not Using Precompiled Headers",
     "value": "NotUsing",
     "flags": []

Templates/MSBuild/FlagTables/v140_CL.json

@@ -422,7 +422,7 @@
   },
   {
     "name": "PrecompiledHeader",
-    "switch": "",
+    "switch": "Y-",
     "comment": "Not Using Precompiled Headers",
     "value": "NotUsing",
     "flags": []

Templates/MSBuild/FlagTables/v141_CL.json

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