[AST][NFC] Small doc update for DeclContext

Factored out from https://reviews.llvm.org/D49729
following @erichkeane comments.

* Add missing classes in the list of classes
  deriving directly from DeclContext.
* Move the friend declarations together and
  add a comment for why they are required.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D49790

llvm-svn: 338887

clang/include/clang/AST/DeclBase.h

@@ -1250,16 +1250,29 @@ public:
 /// that directly derive from DeclContext are mentioned, not their subclasses):
 ///
 ///   TranslationUnitDecl
+///   ExternCContext
 ///   NamespaceDecl
-///   FunctionDecl
 ///   TagDecl
+///   OMPDeclareReductionDecl
+///   FunctionDecl
 ///   ObjCMethodDecl
 ///   ObjCContainerDecl
 ///   LinkageSpecDecl
 ///   ExportDecl
 ///   BlockDecl
-///   OMPDeclareReductionDecl
+///   CapturedDecl
 class DeclContext {
+  /// For makeDeclVisibleInContextImpl
+  friend class ASTDeclReader;
+  /// For reconcileExternalVisibleStorage, CreateStoredDeclsMap,
+  /// hasNeedToReconcileExternalVisibleStorage
+  friend class ExternalASTSource;
+  /// For CreateStoredDeclsMap
+  friend class DependentDiagnostic;
+  /// For hasNeedToReconcileExternalVisibleStorage,
+  /// hasLazyLocalLexicalLookups, hasLazyExternalLexicalLookups
+  friend class ASTWriter;
+
   // We use uint64_t in the bit-fields below since some bit-fields
   // cross the unsigned boundary and this breaks the packing.
 
@@ -1716,10 +1729,6 @@ protected:
                   "BlockDeclBitfields is larger than 8 bytes!");
   };
 
-  friend class ASTDeclReader;
-  friend class ASTWriter;
-  friend class ExternalASTSource;
-
   /// FirstDecl - The first declaration stored within this declaration
   /// context.
   mutable Decl *FirstDecl = nullptr;
@@ -2398,8 +2407,6 @@ private:
     DeclContextBits.HasLazyExternalLexicalLookups = HasLELL;
   }
 
-  friend class DependentDiagnostic;
-
   void reconcileExternalVisibleStorage() const;
   bool LoadLexicalDeclsFromExternalStorage() const;