VS: Populate std:c{11,17} flag table entries for v142

The `-std:c11` option added by commit f7347f28c7 (MSVC: Record support
for C11 and c_restrict, 2020-08-09, v3.18.2~9^2) needs this flag table
entry to map in the VS IDE properly.

Issue: #21069

Templates/MSBuild/FlagTables/v142_CL.json

@@ -454,6 +454,20 @@
     "value": "stdcpplatest",
     "flags": []
   },
+  {
+    "name": "LanguageStandard_C",
+    "switch": "std:c11",
+    "comment": "ISO C11 Standard",
+    "value": "stdc11",
+    "flags": []
+  },
+  {
+    "name": "LanguageStandard_C",
+    "switch": "std:c17",
+    "comment": "ISO C17 (2018) Standard",
+    "value": "stdc17",
+    "flags": []
+  },
   {
     "name": "PrecompiledHeader",
     "switch": "Yc",