[AST] Stash an archetype builder in an "incomplete" generic environment.

This eliminates the really gross registration of archetype builders
within the ASTContext, and is another little step toward lazily
constructing archetypes.

lib/Serialization/Deserialization.cpp

@@ -952,8 +952,7 @@ GenericEnvironment *ModuleFile::readGenericEnvironment(
   assert(!interfaceToArchetypeMap.empty() &&
          "no archetypes in generic function?");
 
-  return GenericEnvironment::get(getContext(), signature,
-                                 interfaceToArchetypeMap);
+  return GenericEnvironment::get(signature, interfaceToArchetypeMap);
 }
 
 GenericEnvironment *ModuleFile::maybeReadGenericEnvironment() {