Sema: Don't dyn_cast a null pointer in CheckUsingDeclQualifier

This code was written with the intent that a pointer could be null but
we dyn_cast'd it anyway.  Change the dyn_cast to a dyn_cast_or_null.

This fixes PR21933.

llvm-svn: 224411
diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index b2a765b..96149c5 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -8159,7 +8159,7 @@
     // If we weren't able to compute a valid scope, it must be a
     // dependent class scope.
     if (!NamedContext || NamedContext->isRecord()) {
-      auto *RD = dyn_cast<CXXRecordDecl>(NamedContext);
+      auto *RD = dyn_cast_or_null<CXXRecordDecl>(NamedContext);
       if (RD && RequireCompleteDeclContext(const_cast<CXXScopeSpec&>(SS), RD))
         RD = nullptr;