[AST] Small doc update for DeclContext

[bricci]

Summary

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.

[bricci]

Re-added the "friend class ASTWriter" after making
hasNeedToReconcileExternalVisibleStorage, hasLazyLocalLexicalLookups
and hasLazyExternalLexicalLookups in DeclContext private.

This match the original situation where HasNeedToReconcileExternalVisibleStorage,
HasLazyLocalLexicalLookups and HasLazyLocalLexicalLookups in DeclContext
were private. This now depends on https://reviews.llvm.org/D49729

include/clang/AST/DeclBase.h

 /// 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.
 

                   "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;

     DeclContextBits.HasLazyExternalLexicalLookups = HasLELL;
   }
 
-  friend class DependentDiagnostic;
-
   void reconcileExternalVisibleStorage() const;
   bool LoadLexicalDeclsFromExternalStorage() const;