[Macros] Add a test ensuring that we print expanded macro definitions

This should complete the implementation of expanded macro
definitions, rdar://104044325.

Author: DougGregor

test/ModuleInterface/macros.swift

@@ -15,6 +15,11 @@
 // CHECK-NEXT: #endif
 @freestanding(expression) public macro publicStringify<T>(_ value: T) -> (T, String) = #externalMacro(module: "SomeModule", type: "StringifyMacro")
 
+@freestanding(expression) public macro labeledStringify<T>(_ value: T, label: String) -> (T, String) = #externalMacro(module: "SomeModule", type: "StringifyMacro")
+
+// CHECK: @freestanding(expression) public macro unlabeledStringify<T>(_ value: T, label: Swift.String) -> (T, Swift.String) = #labeledStringify(value, label: "default label")
+@freestanding(expression) public macro unlabeledStringify<T>(_ value: T, label: String) -> (T, String) = #labeledStringify(value, label: "default label")
+
 // CHECK: #if compiler(>=5.3) && $Macros
 // CHECK-NEXT: #if $FreestandingExpressionMacros
 // CHECK: @freestanding(expression) public macro publicLine<T>() -> T = #externalMacro(module: "SomeModule", type: "Line") where T : Swift.ExpressibleByIntegerLiteral