Make SyntaxTransformVisitor SPI

We want to remove this API, mark it SPI so the
remaining clients can be weaned off.

Author: hamishknight

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

@@ -16,7 +16,12 @@ import SyntaxSupport
 import Utils
 
 let syntaxTransformFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
-  try! ProtocolDeclSyntax("public protocol SyntaxTransformVisitor") {
+  try! ProtocolDeclSyntax(
+    """
+    @_spi(SyntaxTransformVisitor)
+    public protocol SyntaxTransformVisitor
+    """
+  ) {
     DeclSyntax("associatedtype ResultType = Void")
 
     DeclSyntax("func visitAny(_ node: Syntax) -> ResultType")

Tests/SwiftParserTest/SyntaxTransformVisitorTest.swift

@@ -12,7 +12,7 @@
 
 import XCTest
 import SwiftParser
-import SwiftSyntax
+@_spi(SyntaxTransformVisitor) import SwiftSyntax
 
 final class SyntaxTransformVisitorTest: XCTestCase {
   public func testFunctionCounter() {