C#: DB upgrade script and stats.

Author: calumgrant

csharp/ql/src/semmlecode.csharp.dbscheme.stats

@@ -836,6 +836,10 @@
 <v>0</v>
 </e>
 <e>
+<k>@suppress_nullable_warning_expr</k>
+<v>500</v>
+</e>
+<e>
 <k>@xmldtd</k>
 <v>2</v>
 </e>