[NFC] [Serialization] Extract the functionality of merging decls from ASTReaderDecl (#103022)

Currently we're merging the decls in ASTReaderDecl. But it is not so
convinient if we want to merge things in ASTReader.

This patch extract the funcitonality of merging decls from ASTReaderDecl
to a new class, ASTReaderMerger. Then it will be easier to merge decls
in ASTReader.

This may help the readability slightly too.

Author: ChuanqiXu9

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;