[Test] Add test for already-fixed macro example.

This used to cause a circular reference and crash. No more, but make
sure we don't regress it. From rdar://109280926.

Author: DougGregor

test/Macros/macro_expand.swift

@@ -418,3 +418,10 @@ func testFreestandingClosureNesting() {
 func testLocalVarsFromDeclarationMacros() {
   #varValue
 }
+
+// Variadic macro
+@freestanding(declaration, names: arbitrary) macro emptyDecl(_: String...) = #externalMacro(module: "MacroDefinition", type: "EmptyDeclarationMacro")
+
+struct TakesVariadic {
+  #emptyDecl("foo", "bar")
+}