C#: Implement nullable warning suppression expressions.

csharp/ql/src/semmlecode.csharp.dbscheme

@@ -987,6 +987,7 @@ case @expr.kind of
 | 117 = @positional_pattern_expr
 | 118 = @switch_case_expr
 | 119 = @assign_coalesce_expr
+| 120 = @suppress_nullable_warning_expr
 ;
 
 @switch = @switch_stmt | @switch_expr;