C#: Do not clear content of reference type struct fields.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -2528,6 +2528,7 @@ private predicate clearsCont(Node n, Content c) {
     a.getType() = s and
     f = s.getAField() and
     c.(FieldContent).getField() = f.getUnboundDeclaration() and
+    not f.getType().isRefType() and
     not f.isRef()
   )
   or