[Sema] Fixed assert failure in template code analyzer

[kevgs]

Summary

Fix for this bug https://llvm.org/bugs/show_bug.cgi?id=27312

lib/Sema/SemaAccess.cpp

for (SmallVectorImpl<CXXRecordDecl*>::const_iterator
 
     // If the template names don't match, it can't be a dependent
     // match.
     if (CTD->getDeclName() != Friend->getDeclName())
       continue;
 
     // If the class's context can't instantiate to the friend's
     // context, it can't be a dependent match.
-    if (!MightInstantiateTo(S, CTD->getDeclContext(),
-                            Friend->getDeclContext()))
+    if (Friend->getDeclContext()->isDependentContext() ||
+        !MightInstantiateTo(S, CTD->getDeclContext(), Friend->getDeclContext()))
       continue;
 
     // Otherwise, it's a dependent match.
     OnFailure = AR_dependent;
   }
 
   return OnFailure;
 }

test/SemaTemplate/crash-bug-27258.cpp

+// RUN: %clang_cc1 -fsyntax-only %s -verify
+template <class T>
+struct AA {
+  template <T>
+  struct B {};
+};
+
+class C {
+  int i; // expected-note{{implicitly declared private here}}
+  template <class T>
+  template <T>
+  friend struct AA<T>::B;
+};
+
+struct A {
+  template <class T>
+  struct B {
+    void f() {
+      C c;
+      c.i; // expected-error{{'i' is a private member of 'C'}} expected-warning{{expression result unused}}
+    }
+  };
+};