[AST] Get aliased type info from an aliased TemplateSpecialization.

Summary:
Previously the TemplateSpecialization instance for 'template_alias', in the example below, returned the type info of the  canonical type (int).  This ignored the type alias if the template type happen to be aliased. 

Before this patch, the assert would trigger with an  alignment of 4:
```
typedef int __attribute__(( aligned( 16 ) )) aligned_int;
template < typename >
using template_alias = aligned_int;
static_assert( alignof( template_alias<void>) == 16, "" );
```

This patch checks if the TemplateSpecialization type has an alias, and if so will return the type information for the aliased type, else the canonical type's info is returned (original behavior).  I believe that this is the desired behavior.  

Reviewers: aaron.ballman, rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D54048

llvm-svn: 346146

clang/include/clang/AST/Type.h

@@ -4901,7 +4901,9 @@ public:
     return !isDependentType() || isCurrentInstantiation() || isTypeAlias();
   }
 
-  QualType desugar() const { return getCanonicalTypeInternal(); }
+  QualType desugar() const {
+    return isTypeAlias() ? getAliasedType() : getCanonicalTypeInternal();
+  }
 
   void Profile(llvm::FoldingSetNodeID &ID, const ASTContext &Ctx) {
     Profile(ID, Template, template_arguments(), Ctx);

clang/test/SemaCXX/alignof.cpp

@@ -97,3 +97,8 @@ struct S {
   typedef __attribute__((aligned(N))) int Field[sizeof(N)]; // expected-error {{requested alignment is dependent but declaration is not dependent}}
 };
 }
+
+typedef int __attribute__((aligned(16))) aligned_int;
+template <typename>
+using template_alias = aligned_int;
+static_assert(alignof(template_alias<void>) == 16, "Expected alignment of 16" );