[ParseableInterface] Add test for multi-line attribute message

Author: Harlan Haskins

test/ParseableInterface/multiline-availability-message.swift

@@ -0,0 +1,21 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -typecheck -module-name Test -emit-parseable-module-interface-path %t/Test.swiftinterface %s
+// RUN: %FileCheck %s < %t/Test.swiftinterface
+// RUN: %target-swift-frontend -build-module-from-parseable-interface %t/Test.swiftinterface -o %t/Test.swiftmodule
+// RUN: %target-swift-frontend -emit-module -o /dev/null -merge-modules -emit-parseable-module-interface-path - %t/Test.swiftmodule -module-name Test | %FileCheck %s
+
+// CHECK: @available(*, unavailable, message: "First line.\nAnother line!")
+// CHECK: public func catastrophicFunction()
+@available(*,
+  unavailable,
+  message: """
+    First line.
+    Another line!
+    """)
+public func catastrophicFunction() {}
+
+// CHECK: @available(*, unavailable, message: "\n    First line.\n    Another line!")
+// CHECK: public func catastrophicFunction2()
+@available(*, unavailable, message: "\n    First line.\n    Another line!")
+public func catastrophicFunction2() {}