Add a couple of missing SPI imports

Author: hamishknight

CodeGeneration/Sources/generate-swift-syntax/templates/swiftparserdiagnostics/ChildNameForDiagnosticsFile.swift

@@ -16,7 +16,7 @@ import SyntaxSupport
 import Utils
 
 let childNameForDiagnosticFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
- DeclSyntax("import SwiftSyntax")
+ DeclSyntax("@_spi(ExperimentalLanguageFeatures) import SwiftSyntax")
 
  try! FunctionDeclSyntax(
  "private func childNameForDiagnostics(_ keyPath: AnyKeyPath) -> String?"

CodeGeneration/Sources/generate-swift-syntax/templates/swiftsyntaxbuilder/BuildableNodesFile.swift

@@ -16,7 +16,7 @@ import SyntaxSupport
 import Utils
 
 let buildableNodesFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
- DeclSyntax("import SwiftSyntax")
+ DeclSyntax("@_spi(ExperimentalLanguageFeatures) import SwiftSyntax")
 
  for node in SYNTAX_NODES.compactMap(\.layoutNode) {
  let type = node.type

Sources/SwiftParserDiagnostics/generated/ChildNameForDiagnostics.swift

@@ -12,7 +12,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-import SwiftSyntax
+@_spi(ExperimentalLanguageFeatures) import SwiftSyntax
 
 private func childNameForDiagnostics(_ keyPath: AnyKeyPath) -> String? {
  switch keyPath {

Sources/SwiftSyntaxBuilder/generated/BuildableNodes.swift

@@ -12,7 +12,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-import SwiftSyntax
+@_spi(ExperimentalLanguageFeatures) import SwiftSyntax
 
 extension AccessorDeclSyntax {
  /// A convenience initializer that allows initializing syntax collections using result builders