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/ModuleFormat.h

@@ -482,7 +482,6 @@ namespace decls_block {
     TypeIDField, // underlying type
     TypeIDField, // interface type
     BCFixed<1>  // implicit flag
-    // Trailed by the conformance info (if any).
   >;
 
   using GenericTypeParamDeclLayout = BCRecordLayout<