[NFC] [Serialization] Extract the functionality of merging decls from ASTReaderDecl

clang/include/clang/AST/DeclBase.h

@@ -324,6 +324,7 @@ class alignas(8) Decl {
   static bool StatisticsEnabled;
 
 protected:
+  friend class ASTDeclMerger;
   friend class ASTDeclReader;
   friend class ASTDeclWriter;
   friend class ASTNodeImporter;

clang/include/clang/AST/DeclCXX.h

@@ -256,6 +256,7 @@ class CXXBaseSpecifier {
 
 /// Represents a C++ struct/union/class.
 class CXXRecordDecl : public RecordDecl {
+  friend class ASTDeclMerger;
   friend class ASTDeclReader;
   friend class ASTDeclWriter;
   friend class ASTNodeImporter;

clang/include/clang/AST/DeclObjC.h

@@ -1747,6 +1747,7 @@ class ObjCInterfaceDecl : public ObjCContainerDecl
   static bool isKnownExtension(ObjCCategoryDecl *Cat);
 
 public:
+  friend class ASTDeclMerger;
   friend class ASTDeclReader;
   friend class ASTDeclWriter;
   friend class ASTReader;
@@ -2134,6 +2135,7 @@ class ObjCProtocolDecl : public ObjCContainerDecl,
   void setHasODRHash(bool HasHash);
 
 public:
+  friend class ASTDeclMerger;
   friend class ASTDeclReader;
   friend class ASTDeclWriter;
   friend class ASTReader;

clang/include/clang/AST/Redeclarable.h

@@ -191,6 +191,7 @@ class Redeclarable {
   }
 
 public:
+  friend class ASTDeclMerger;
   friend class ASTDeclReader;
   friend class ASTDeclWriter;
   friend class IncrementalParser;

clang/include/clang/Serialization/ASTReader.h

@@ -378,6 +378,7 @@ class ASTReader
 {
 public:
   /// Types of AST files.
+  friend class ASTDeclMerger;
   friend class ASTDeclReader;
   friend class ASTIdentifierIterator;
   friend class ASTRecordReader;