AST: Robustness fix

Author: slavapestov

lib/AST/Decl.cpp

@@ -2404,6 +2404,10 @@ ObjCClassKind ClassDecl::checkObjCAncestry() const {
     if (!CD->hasSuperclass())
       break;
     CD = CD->getSuperclass()->getClassOrBoundGenericClass();
+    // If we don't have a valid class here, we should have diagnosed
+    // elsewhere.
+    if (!CD)
+      break;
   }
 
   if (!isObjC)

validation-test/compiler_crashers/28367-swift-declcontext-isgenericcontext.swift renamed to validation-test/compiler_crashers_fixed/28367-swift-declcontext-isgenericcontext.swift

@@ -5,5 +5,5 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -typecheck
+// RUN: not %target-swift-frontend %s -typecheck
 let a{{}protocol A{class A:d class d<U:A>:A