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.

include/swift/AST/Decl.h

@@ -1482,18 +1482,11 @@ public:
   /// moves the trailing where clause into the generic parameter list.
   TrailingWhereClause *TrailingWhere = nullptr;
 
-  /// The generic signature or environment of this declaration.
-  ///
-  /// When this declaration stores only a signature, the generic
-  /// environment will be lazily loaded.
-  mutable llvm::PointerUnion<GenericSignature *, GenericEnvironment *>
-    GenericSigOrEnv;
+  /// The generic signature of this declaration.
+  GenericSignature *GenericSig = nullptr;
 };
 
 class GenericContext : private _GenericContext, public DeclContext {
-  /// Lazily populate the generic environment.
-  GenericEnvironment *getLazyGenericEnvironmentSlow() const;
-
 protected:
   GenericContext(DeclContextKind Kind, DeclContext *Parent)
     : _GenericContext(), DeclContext(Kind, Parent) { }
@@ -1542,18 +1535,12 @@ public:
   /// Retrieve the generic requirements.
   ArrayRef<Requirement> getGenericRequirements() const;
 
-  /// Set a lazy generic environment.
-  void setLazyGenericEnvironment(LazyMemberLoader *lazyLoader,
-                                 GenericSignature *genericSig,
-                                 uint64_t genericEnvData);
-
-  /// Whether this generic context has a lazily-created generic environment
-  /// that has not yet been constructed.
-  bool hasLazyGenericEnvironment() const;
-
   /// Set the generic context of this context.
   void setGenericEnvironment(GenericEnvironment *genericEnv);
 
+  /// Set the generic signature of this context.
+  void setGenericSignature(GenericSignature *genericSig);
+
   /// Retrieve the position of any where clause for this context's
   /// generic parameters.
   SourceRange getGenericTrailingWhereClauseSourceRange() const;