C#: Update read steps for tuples as they are now part of the PatternDefinition.

Author: michaelnebel

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

@@ -2242,8 +2242,8 @@ private predicate readContentStep(Node node1, Content c, Node node2) {
     )
     or
     // item = variable in node1 = (..., variable, ...) in a case/is var (..., ...)
-    isPatternExprDescendant(te) and
-    exists(AssignableDefinitions::LocalVariableDefinition lvd |
+    te instanceof TuplePatternExpr and
+    exists(AssignableDefinitions::PatternDefinition lvd |
       node2.(AssignableDefinitionNode).getDefinition() = lvd and
       lvd.getDeclaration() = item
     )