PR-2107 follow-up: avoid sharing AST nodes (eclipse-jdt#4127)

PR 2107 copies a reference to an AST node (local declaration) which
implies that the same node will be visited twice for all phases.

Let's play safe and move instead of copy in order to maintain that AST
is indeed a tree :)

Author: stephan-herrmann

org.eclipse.jdt.core/codeassist/org/eclipse/jdt/internal/codeassist/complete/CompletionParser.java

@@ -697,6 +697,7 @@ protected void attachOrphanCompletionNode(){
 				//  if (a instanceof List l) { l.is| Object // https://github.com/eclipse-jdt/eclipse.jdt.core/issues/2106
 				if (ifStatement.condition instanceof InstanceOfExpression iof && iof.pattern instanceof TypePattern pattern) {
 					this.currentElement.add(pattern.local, 0);
+					iof.pattern = null;
 				}
 				this.currentElement = this.currentElement.add(ifStatement, 0);
 			}