Serialization: Preserve identity of opened generic environments

We used to create a new environment for each opened archetype, which is
incorrect when deserializing a nested type of another opened archetype.

lib/Serialization/ModuleFile.h

@@ -259,6 +259,9 @@ private:
   /// Generic signatures referenced by this module.
   MutableArrayRef<Serialized<GenericSignature>> GenericSignatures;
 
+  /// Generic environments referenced by this module.
+  MutableArrayRef<Serialized<GenericEnvironment *>> GenericEnvironments;
+
   /// Substitution maps referenced by this module.
   MutableArrayRef<Serialized<SubstitutionMap>> SubstitutionMaps;
 
@@ -862,6 +865,10 @@ public:
   llvm::Expected<GenericSignature>
   getGenericSignatureChecked(serialization::GenericSignatureID ID);
 
+  /// Returns the generic environment for the given ID or the first error.
+  llvm::Expected<GenericEnvironment *>
+  getGenericEnvironmentChecked(serialization::GenericEnvironmentID ID);
+
   /// Returns the substitution map for the given ID, deserializing it if
   /// needed.
   SubstitutionMap getSubstitutionMap(serialization::SubstitutionMapID id);