[SyntaxNode] Make 'static var structure' immutable stored properties

Returnning array literal from computed property allocates and
initializes the array buffer every time it's called, and they are
deallocated after use.
Make them stored properties so they stay in memory.

Author: rintaro

CodeGeneration/Sources/generate-swift-syntax/templates/swiftsyntax/SyntaxNodesFile.swift

@@ -199,17 +199,14 @@ func syntaxNode(nodesStartingWith: [Character]) -> SourceFileSyntax {
           }
         }
 
-        try! VariableDeclSyntax("public static var structure: SyntaxNodeStructure") {
-          let layout = ArrayExprSyntax {
-            for child in node.children {
-              ArrayElementSyntax(
-                expression: ExprSyntax(#"\Self.\#(child.varOrCaseName)"#)
-              )
-            }
+        let layout = ArrayExprSyntax {
+          for child in node.children {
+            ArrayElementSyntax(
+              expression: ExprSyntax(#"\Self.\#(child.varOrCaseName)"#)
+            )
           }
-
-          StmtSyntax("return .layout(\(layout))")
         }
+        "public static let structure: SyntaxNodeStructure = .layout(\(layout))"
       }
     }
   }