[ASTGen] Update for SPI in swift-syntax

All symbols in `SwiftSyntaxMacroExpansion` and
`SwiftCompilerPluginMessageHandling` are now SPI.

Author: rintaro

lib/ASTGen/Sources/ASTGen/Macros.swift

@@ -12,12 +12,12 @@
 
 import ASTBridging
 import BasicBridging
-@_spi(ExperimentalLanguageFeature) import SwiftCompilerPluginMessageHandling
+@_spi(PluginMessage) @_spi(ExperimentalLanguageFeature) import SwiftCompilerPluginMessageHandling
 import SwiftDiagnostics
 import SwiftOperators
 import SwiftSyntax
 import SwiftSyntaxBuilder
-@_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacroExpansion
+@_spi(MacroExpansion) @_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacroExpansion
 @_spi(ExperimentalLanguageFeature) import SwiftSyntaxMacros
 
 /// Describes a macro that has been "exported" to the C++ part of the

lib/ASTGen/Sources/ASTGen/PluginHost.swift

@@ -12,7 +12,7 @@
 
 import ASTBridging
 import BasicBridging
-import SwiftCompilerPluginMessageHandling
+@_spi(PluginMessage) import SwiftCompilerPluginMessageHandling
 import SwiftSyntax
 import swiftLLVMJSON
 

tools/swift-plugin-server/Sources/swift-plugin-server/swift-plugin-server.swift

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-import SwiftCompilerPluginMessageHandling
+@_spi(PluginMessage) import SwiftCompilerPluginMessageHandling
 import SwiftSyntaxMacros
 import swiftLLVMJSON
 import CSwiftPluginServer