[serialization] Add support for UnboundGenericTypes.

I had previously thought these didn't appear in public decls, but they're
used when you extend a generic class without generic arguments.

Swift SVN r6187

lib/Serialization/ModuleFormat.h

@@ -226,6 +226,7 @@ namespace decls_block {
     ARRAY_SLICE_TYPE,
     ARRAY_TYPE,
     REFERENCE_STORAGE_TYPE,
+    UNBOUND_GENERIC_TYPE,
 
     TYPE_ALIAS_DECL = 100,
     STRUCT_DECL,
@@ -391,6 +392,12 @@ namespace decls_block {
     TypeIDField      // implementation type
   >;
 
+  using UnboundGenericTypeLayout = BCRecordLayout<
+    UNBOUND_GENERIC_TYPE,
+    DeclIDField, // generic decl
+    TypeIDField  // parent
+  >;
+
 
   using TypeAliasLayout = BCRecordLayout<
     TYPE_ALIAS_DECL,