[Clang Importer] Test for deferred module imports, rdar://30615193

Author: graydon

test/ClangImporter/MixedSource/Inputs/defer-supermodule-import/Some-Bridging-Header.h

@@ -0,0 +1,7 @@
+#import <Some/Submodule.h>
+
+@interface Thing
++ (void)fn:(KLASS*)arg;
+@end
+
+void bar(TYPE t);

test/ClangImporter/MixedSource/Inputs/defer-supermodule-import/Some.framework/Headers/Some.h

@@ -0,0 +1,2 @@
+typedef int TYPE;
+@class KLASS;

test/ClangImporter/MixedSource/Inputs/defer-supermodule-import/Some.framework/Headers/Submodule.h

@@ -0,0 +1 @@
+int foo();

test/ClangImporter/MixedSource/Inputs/defer-supermodule-import/Some.framework/Modules/module.modulemap

@@ -0,0 +1,8 @@
+framework module Some  {
+  header "Some.h"
+  export *
+  module Submodule {
+    header "Submodule.h"
+    export *
+  }
+}

test/ClangImporter/MixedSource/defer-supermodule-import.swift

@@ -0,0 +1,21 @@
+// RUN: not %target-swift-frontend -F %S/Inputs/defer-supermodule-import -import-objc-header %S/Inputs/defer-supermodule-import/Some-Bridging-Header.h -typecheck %s 2>&1 | %FileCheck -check-prefix=HEADER-ERROR %s
+// HEADER-ERROR: Some-Bridging-Header.h:4:13: error: expected a type
+// HEADER-ERROR: Some-Bridging-Header.h:7:10: error: declaration of 'TYPE' must be imported from module 'Some' before it is required
+// REQUIRES: objc_interop
+
+// The bug we're testing here is that:
+//
+//  - Given a supermodule defining some types
+//  - Given a submodule of that supermodule
+//  - Given an _erroneous_ bridging header that imports the _submodule_ and tries
+//    to use the _supermodule's_ types
+//
+// That we emit an error. Previously we did not: Swift's "implicit supermodule
+// import" rule would fire _eagerly_, so by the time the submodule import was
+// complete the supermodule was also imported and the bridging header would pass
+// through without flagging the error. This made it possible to write textual
+// bridging headers that would not, themselves, be considered valid objc code as
+// far as clang (or more importantly: clang's PCH-generating pass) was
+// concerned.
+
+print(bar(foo()))