Disallow inheritance clauses for concrete typealiases.

typealias MyInt: ForwardIndex = Int

There is no real reason to allow this; it's just a static_assert that Int
conforms to ForwardIndex, which would be better spelled some other way.

This only applies to concrete typealiases, i.e. those that simply alias an
underlying type. Associated types can still have both inheritance clauses
and a (default) underlying type.

Swift SVN r11481

lib/Serialization/Serialization.cpp

@@ -1080,6 +1080,8 @@ void Serializer::writeDecl(const Decl *D) {
   case DeclKind::TypeAlias: {
     auto typeAlias = cast<TypeAliasDecl>(D);
     assert(!typeAlias->isObjC() && "ObjC typealias is not meaningful");
+    assert(typeAlias->getProtocols().empty() &&
+           "concrete typealiases cannot have protocols");
     checkAllowedAttributes<>(typeAlias);
 
     const Decl *DC = getDeclForContext(typeAlias->getDeclContext());
@@ -1095,9 +1097,6 @@ void Serializer::writeDecl(const Decl *D) {
                                 addTypeRef(underlying),
                                 addTypeRef(typeAlias->getInterfaceType()),
                                 typeAlias->isImplicit());
-
-    writeConformances(typeAlias->getProtocols(), typeAlias->getConformances(),
-                      typeAlias, DeclTypeAbbrCodes);
     break;
   }