AST: GenericContexts store a GenericSignature instead of a GenericEnvironment

This eliminates the entire 'lazy generic environment' concept;
essentially, all generic environments are now lazy, and since
each signature has exactly one environment, their construction
no longer needs to be co-ordinated with deserialization.

lib/Serialization/ModuleFile.h

@@ -554,11 +554,6 @@ private:
   void readGenericRequirements(SmallVectorImpl<Requirement> &requirements,
                                llvm::BitstreamCursor &Cursor);
 
-  /// Set up a (potentially lazy) generic environment for the given type,
-  /// function or extension.
-  void configureGenericEnvironment(GenericContext *genericDecl,
-                                   serialization::GenericSignatureID envID);
-
   /// Populates the protocol's default witness table.
   ///
   /// Returns true if there is an error.
@@ -789,9 +784,6 @@ public:
   virtual void finishNormalConformance(NormalProtocolConformance *conformance,
                                        uint64_t contextData) override;
 
-  GenericEnvironment *loadGenericEnvironment(const DeclContext *decl,
-                                             uint64_t contextData) override;
-
   void
   loadRequirementSignature(const ProtocolDecl *proto, uint64_t contextData,
                            SmallVectorImpl<Requirement> &requirements) override;