VS: Populate `-Qspectre-` flag table entry for v142

Rather than the change from commit 584ad067ba (VS: Fix flag table entry
for -Qspectre, 2018-11-26), update the v142 flag table with an actual
flag for the negative form of the flag.  Switch the entry order so that
the longer negative form is first.

Templates/MSBuild/FlagTables/v142_CL.json

@@ -421,16 +421,16 @@
   },
   {
     "name": "SpectreMitigation",
-    "switch": "Qspectre",
-    "comment": "Enabled",
-    "value": "Spectre",
+    "switch": "Qspectre-",
+    "comment": "Spectre mitigations disabled",
+    "value": "false",
     "flags": []
   },
   {
     "name": "SpectreMitigation",
-    "switch": "",
-    "comment": "Disabled",
-    "value": "false",
+    "switch": "Qspectre",
+    "comment": "Spectre mitigations enabled",
+    "value": "Spectre",
     "flags": []
   },
   {