Merge pull request #25395 from harlanhaskins/extensify-5.1

[5.1] [Sema] Bail early in exportability checker if there's no extended type

Author: Harlan Haskins

lib/Sema/TypeCheckAccess.cpp

@@ -1873,7 +1873,12 @@ class ExportabilityChecker : public DeclVisitor<ExportabilityChecker> {
   }
 
   void visitExtensionDecl(ExtensionDecl *ED) {
-    if (shouldSkipChecking(ED->getExtendedNominal()))
+    auto extendedType = ED->getExtendedNominal();
+    // TODO: Sometimes we have an extension that is marked valid but has no
+    //       extended type. Assert, just in case we see it while testing, but
+    //       don't crash. rdar://50401284
+    assert(extendedType && "valid extension with no extended type?");
+    if (!extendedType || shouldSkipChecking(extendedType))
       return;
 
     // FIXME: We should allow conforming to implementation-only protocols,