[Macros] PluginMessages aren't @_spi anymore

rdar://111748087
(cherry picked from commit 928c94f)

Author: rintaro

lib/ASTGen/Sources/ASTGen/Macros.swift

@@ -18,7 +18,7 @@ import SwiftSyntax
 import SwiftSyntaxBuilder
 import SwiftSyntaxMacros
 import SwiftSyntaxMacroExpansion
-@_spi(PluginMessage) import SwiftCompilerPluginMessageHandling
+import SwiftCompilerPluginMessageHandling
 
 extension SyntaxProtocol {
   func token(at position: AbsolutePosition) -> TokenSyntax? {

lib/ASTGen/Sources/ASTGen/PluginHost.swift

@@ -14,7 +14,7 @@ import CASTBridging
 import CBasicBridging
 import SwiftSyntax
 import swiftLLVMJSON
-@_spi(PluginMessage) import SwiftCompilerPluginMessageHandling
+import SwiftCompilerPluginMessageHandling
 
 enum PluginError: String, Error, CustomStringConvertible {
   case stalePlugin = "plugin is stale"