Java: ContainerFlow: RValue -> Expr

While most flow for a qualifierToArgumentStep goes through a variable use
this is not always the case. Therefore it is best to remove the restriction
to RValue to allow taint steps to use postupdate nodes.

See also: ba86dea657

java/ql/src/semmle/code/java/dataflow/internal/ContainerFlow.qll

@@ -180,7 +180,7 @@ private predicate qualifierToMethodStep(Expr tracked, MethodAccess sink) {
   tracked = sink.getQualifier()
 }
 
-private predicate qualifierToArgumentStep(Expr tracked, RValue sink) {
+private predicate qualifierToArgumentStep(Expr tracked, Expr sink) {
   exists(MethodAccess ma, CollectionMethod method |
     method = ma.getMethod() and
     (