[cxx-interop] Fix a crash when exposing @objc Swift classes

rdar://154252454

Author: Gabor Horvath

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -596,7 +596,8 @@ class ModuleWriter {
         else if (isa<StructDecl>(TD) && NTD->hasClangNode())
           emitReferencedClangTypeMetadata(NTD);
         else if (const auto *cd = dyn_cast<ClassDecl>(TD))
-          if (cd->isObjC() || cd->isForeignReferenceType())
+          if ((cd->isObjC() && cd->getClangDecl()) ||
+              cd->isForeignReferenceType())
             emitReferencedClangTypeMetadata(NTD);
       } else if (auto TAD = dyn_cast<TypeAliasDecl>(TD)) {
         if (TAD->hasClangNode())

test/Interop/ObjCToSwiftToObjCxx/bridge-objc-types-back-to-objcxx.swift

@@ -88,10 +88,17 @@ public func retObjCClassArray() -> [ObjCKlass] {
     return []
 }
 
+public class KVOCookieMonster {
+   public static func += (lhs: KVOCookieMonster, rhs: NSKeyValueObservation) {
+      lhs.cookies.append(rhs)
+   }
+
+   private var cookies = Array<NSKeyValueObservation>()
+}
+
 // CHECK: @interface HasBlockField : NSObject
 // CHECK: @property (nonatomic, copy) void (^ _Nullable foo)(ObjCKlassState);
 // CHECK: @end
-
 // CHECK: SWIFT_EXTERN id <ObjCProtocol> _Nonnull $s9UseObjCTy03retB9CProtocolSo0bE0_pyF(void) SWIFT_NOEXCEPT SWIFT_CALL; // retObjCProtocol()
 // CHECK-NEXT: #endif
 // CHECK-NEXT: #if defined(__OBJC__)