[serialization] Remove skeleton for sharing module decl/type IDs.

Clang modules have a clever way to share decl and type IDs, by saying
"this range of IDs comes from this other module". Swift modules have to
be resilient, however, and so this is not a viable solution.

We still use 0 as a special ID for null decls and types, when applicable.

Swift SVN r5323

lib/Serialization/ModuleFormat.h

@@ -173,7 +173,6 @@ namespace index_block {
 
   using OffsetsLayout = BCGenericRecordLayout<
     BCFixed<2>,  // record ID
-    DeclIDField, // first ID in this module
     BCArray<BitOffsetField>
   >;
 }