Canonicalize the variadic template alias with multiple ellipsis.

[logan]

Summary

If the template has two variadic formal parameters, then the type
might not be canonical. We have call ASTContext.getCanonicalType()
to canonicalize the type; otherwise, an assertion failure will be
raised.

This patch fix this issue by adding getCanonicalType() in
TransformTemplateTypeParmType.

lib/Sema/SemaTemplateInstantiate.cpp

if (T->isParameterPack()) {
       }
       
       Arg = getPackSubstitutedTemplateArgument(getSema(), Arg);
     }
     
     assert(Arg.getKind() == TemplateArgument::Type &&
            "Template argument kind mismatch");
 
-    QualType Replacement = Arg.getAsType();
+    QualType Replacement = getSema().Context.getCanonicalType(Arg.getAsType());
 
     // TODO: only do this uniquing once, at the start of instantiation.
     QualType Result
       = getSema().Context.getSubstTemplateTypeParmType(T, Replacement);
     SubstTemplateTypeParmTypeLoc NewTL
       = TLB.push<SubstTemplateTypeParmTypeLoc>(Result);
     NewTL.setNameLoc(TL.getNameLoc());
     return Result;

test/SemaTemplate/alias-templates.cpp

struct base {
     template <typename> struct derived;
   };
   // FIXME: The diagnostics here are terrible.
   template <typename T, typename U, typename V>
   using derived = base<T, U>::template derived<V>; // expected-error {{expected a type}} expected-error {{expected ';'}}
   template <typename T, typename U, typename V>
   using derived2 = ::PR16904::base<T, U>::template derived<V>; // expected-error {{expected a type}} expected-error {{expected ';'}}
 }
+
+namespace VariadicTemplateAlias {
+  template <typename... T> struct tuple;
+  template <typename... T> struct extract_;
+
+  // Note: Both the template alias and the concatenation of variadic template
+  // arguments A and B are required to trigger the assertion failure.
+
+  template <typename... T>
+  using extract = typename extract_<T...>::type;
+
+  template <typename... A, typename... B>
+  inline auto test(tuple<A...>&& xs, B&&... ys) -> extract<A&&..., B...> { }
+}