Add test case for failure seen in source compat suite

Master source compat suite job #4912 failed to build ReactiveKit because Combine does not have an interface for older iOS architectures that were retired before it was released, and a change to module loading diagnosis started to consider switfinterfaces. Add a test case that covers this situation.

Author: beccadax

test/ModuleInterface/can-import.swift

@@ -1,5 +1,7 @@
 // RUN: %empty-directory(%t)
 // RUN: echo 'public func externalFunc() {}' | %target-swift-frontend -typecheck -emit-module-interface-path %t/Library.swiftinterface -
+// RUN: %empty-directory(%t/LibraryWithoutThisArchitecture.swiftmodule)
+// RUN: echo 'public func externalFunc() {}' | %target-swift-frontend -typecheck -emit-module-interface-path %t/LibraryWithoutThisArchitecture.swiftmodule/arm40000-apple-ios.swiftinterface -
 // RUN: %target-swift-frontend -typecheck %s -I %t
 
 #if canImport(Library)
@@ -11,4 +13,8 @@ externalFunc()
 
 #if canImport(LibraryThatDoesNotExist)
 #error("should not return true for library that does not exist")
-#endif
+#endif
+
+#if canImport(LibraryWithoutThisArchitecture)
+#error("should not return true (or diagnose a loader failure) for library that does not exist")
+#endif