Value-preserving methods: handle generics in DataFlowUtil.qll

Co-authored-by: Anders Schack-Mulligen <aschackmull@users.noreply.github.com>

java/ql/src/semmle/code/java/dataflow/FlowSteps.qll

@@ -64,7 +64,6 @@ private class StandardLibraryValuePreservingCallable extends ValuePreservingCall
     )
     or
     this.getDeclaringType()
-        .getSourceDeclaration()
         .getASourceSupertype*()
         .hasQualifiedName("java.util", "Stack") and
     this.hasName("push") and

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

@@ -410,7 +410,7 @@ predicate simpleLocalFlowStep(Node node1, Node node2) {
   summaryStep(node1, node2, "value")
   or
   exists(MethodAccess ma, ValuePreservingCallable c, int argNo |
-    ma.getCallee() = c and c.returnsValue(argNo)
+    ma.getCallee().getSourceDeclaration() = c and c.returnsValue(argNo)
   |
     node2.asExpr() = ma and
     (