[clang] Fix an assertion crash in delayed access check.

[hokein]

Summary

TD-getTemplatedDecl() might not be a DeclContext variant, which can
trigger an assertion inside isa<>.

clang/lib/Sema/SemaAccess.cpp

void Sema::HandleDelayedAccessCheck(DelayedDiagnostic &DD, Decl *D) {
   // However, this does not apply to local extern declarations.
 
   DeclContext *DC = D->getDeclContext();
   if (D->isLocalExternDecl()) {
     DC = D->getLexicalDeclContext();
   } else if (FunctionDecl *FN = dyn_cast<FunctionDecl>(D)) {
     DC = FN;
   } else if (TemplateDecl *TD = dyn_cast<TemplateDecl>(D)) {
+    if (isa<DeclContext>(TD->getTemplatedDecl()))
       DC = cast<DeclContext>(TD->getTemplatedDecl());
   }
 
   EffectiveContext EC(DC);
 
   AccessTarget Target(DD.getAccessData());
 
   if (CheckEffectiveAccess(*this, EC, DD.Loc, Target) == ::AR_inaccessible)
     DD.Triggered = true;

clang/test/SemaCXX/cxx14-access.cpp

+// RUN: %clang_cc1 -fsyntax-only -std=c++14 -verify %s
+
+namespace NoCrashOnDelayedAccessCheck {
+class Foo {
+  class Private; // expected-note {{declared private here}}
+};
+
+struct Bar {};
+
+template <typename T>
+Foo::Private Bar::ABC; // expected-error {{no member named 'ABC' in 'NoCrashOnDelayedAccessCheck::Bar'}} \
+                          expected-error {{'Private' is a private member of}}
+}