Extend lifetime of children in SyntaxCollection initializer

We were already doing this for layout nodes but we forgot to do it for collection nodes.

Author: ahoppen

CodeGeneration/Sources/generate-swiftsyntax/templates/swiftsyntax/SyntaxCollectionsFile.swift

@@ -169,8 +169,15 @@ let syntaxCollectionsFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
         """
         public init(_ children: [Element]) {
           let arena = SyntaxArena()
-          let raw = RawSyntax.makeLayout(kind: SyntaxKind.\(node.varOrCaseName),
-                                         from: children.map { $0.raw }, arena: arena)
+          // Extend the lifetime of children so their arenas don't get destroyed
+          // before they can be added as children of the new arena.
+          let raw = withExtendedLifetime(children) {
+            RawSyntax.makeLayout(
+              kind: SyntaxKind.\(node.varOrCaseName),
+              from: children.map { $0.raw },
+              arena: arena
+            )
+          }
           self.init(SyntaxData.forRoot(raw, rawNodeArena: arena))
         }
         """