Make underlying data structures of SyntaxData.Info classes insted of structs

This will allow us to add more data to `SyntaxData.Info.Root` without increasing the size of `SyntaxData`. It shouldn’t have a big performance impact at the moment since `Root.arena` is never accessed and only exists to keep `arena` a live.

Author: ahoppen

Sources/SwiftSyntax/SyntaxData.swift

@@ -189,17 +189,28 @@ struct AbsoluteRawSyntax {
 struct SyntaxData {
   fileprivate enum Info {
     case root(Root)
-    indirect case nonRoot(NonRoot)
+    case nonRoot(NonRoot)
 
     // For root node.
-    struct Root {
+    class Root {
       var arena: SyntaxArena
+
+      init(arena: SyntaxArena) {
+        self.arena = arena
+      }
     }
 
     // For non-root nodes.
-    struct NonRoot {
+    class NonRoot {
       var parent: SyntaxData
       var absoluteInfo: AbsoluteSyntaxInfo
+      unowned(unsafe) var rootInfo: Root
+
+      init(parent: SyntaxData, absoluteInfo: AbsoluteSyntaxInfo, rootInfo: __shared Root) {
+        self.parent = parent
+        self.absoluteInfo = absoluteInfo
+        self.rootInfo = rootInfo
+      }
     }
   }
 
@@ -209,7 +220,7 @@ struct SyntaxData {
   private var rootInfo: Info.Root {
     switch info {
     case .root(let info): return info
-    case .nonRoot(let info): return info.parent.rootInfo
+    case .nonRoot(let info): return info.rootInfo
     }
   }
 
@@ -274,7 +285,7 @@ struct SyntaxData {
   }
 
   init(_ raw: RawSyntax, parent: SyntaxData, absoluteInfo: AbsoluteSyntaxInfo) {
-    self.init(raw, info: .nonRoot(.init(parent: parent, absoluteInfo: absoluteInfo)))
+    self.init(raw, info: .nonRoot(.init(parent: parent, absoluteInfo: absoluteInfo, rootInfo: parent.rootInfo)))
   }
 
   /// Creates a `SyntaxData` with the provided raw syntax and parent.