Fix a memory corruption bug when modifying SwiftSyntax nodes

When modifying SwiftSyntax nodes that calls `replacingSelf`, we could get into a situation where `arena` gets deallocated before it can be retained by the `forRoot` call.

Throw in a `withExtendedLifetime` call to make sure we don’t loose the arena.

rdar://109860357

Author: ahoppen

Sources/SwiftSyntax/SyntaxData.swift

@@ -311,8 +311,11 @@ struct SyntaxData {
       let newParent = Syntax(parentData)
       return SyntaxData(absoluteRaw.replacingSelf(newRaw, newRootId: parentData.nodeId.rootId), parent: newParent)
     } else {
-      // Otherwise, we're already the root, so return the new root data.
-      return .forRoot(newRaw)
+      // Keep `arena` alive until it is retained by `SyntaxData` produced by `forRoot`.
+      return withExtendedLifetime(arena) { _ in
+        // Otherwise, we're already the root, so return the new root data.
+        return .forRoot(newRaw)
+      }
     }
   }