[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

public:
   }
 };
 
 /// DeclContext - This is used only as base class of specific decl types that
 /// can act as declaration contexts. These decls are (only the top classes
 /// 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.
 
   /// Stores the bits used by DeclContext.
   /// If modified NumDeclContextBit, the ctor of DeclContext and the accessor
   /// methods in DeclContext should be updated appropriately.
   class DeclContextBitfields {
     friend class DeclContext;
union {
     static_assert(sizeof(ObjCContainerDeclBitfields) <= 8,
                   "ObjCContainerDeclBitfields is larger than 8 bytes!");
     static_assert(sizeof(LinkageSpecDeclBitfields) <= 8,
                   "LinkageSpecDeclBitfields is larger than 8 bytes!");
     static_assert(sizeof(BlockDeclBitfields) <= 8,
                   "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;
 
   /// LastDecl - The last declaration stored within this declaration
   /// context. FIXME: We could probably cache this value somewhere
   /// outside of the DeclContext, to reduce the size of DeclContext by
   /// another pointer.
private:
   }
 
   /// If \c true, the external source may have lexical declarations
   /// that are missing from the lookup table.
   void setHasLazyExternalLexicalLookups(bool HasLELL = true) const {
     DeclContextBits.HasLazyExternalLexicalLookups = HasLELL;
   }
 
-  friend class DependentDiagnostic;
-
   void reconcileExternalVisibleStorage() const;
   bool LoadLexicalDeclsFromExternalStorage() const;
 
   /// Makes a declaration visible within this context, but
   /// suppresses searches for external declarations with the same
   /// name.
   ///
   /// Analogous to makeDeclVisibleInContext, but for the exclusive