Fix issue with SPI protocol requirement

Author: beccadax

Sources/SwiftSyntax/SwiftSyntaxCompatibility.swift

@@ -405,6 +405,21 @@ extension GenericArgumentSyntax {
   }
 }
 
+extension FreestandingMacroExpansionSyntax {
+  // SPI protocol requirements require a default implementation, but this should never be used because there should be
+  // no conforming types outside of SwiftSyntax.
+  @_spi(ExperimentalLanguageFeatures)
+  @available(*, deprecated, message: "concrete type should implement its own moduleSelector property")
+  public var moduleSelector: ModuleSelectorSyntax? {
+    get {
+      return nil
+    }
+    set {
+      fatalError("\(Self.self) should implement its own moduleSelector property")
+    }
+  }
+}
+
 extension FunctionCallExprSyntax {
   @_disfavoredOverload
   @available(