[NFC] [Serialization] Extract logics to write special decls from

ASTWriter::WriteASTCore

ASTWriter::WriteASTCore is a big function and slightly hard to read.
This patch extracts the logics to force emitting special declarations
and writing special records for these declarations into member
functions.

This is helpful to improve the readability and also helpful for the
following patch to write special declarations lazily.

clang/include/clang/Serialization/ASTWriter.h

@@ -559,6 +559,8 @@ class ASTWriter : public ASTDeserializationListener,
   void WriteIdentifierTable(Preprocessor &PP, IdentifierResolver &IdResolver,
                             bool IsModule);
   void WriteDeclAndTypes(ASTContext &Context);
+  void PrepareWritingSpecialDecls(Sema &SemaRef);
+  void WriteSpecialDeclRecords(Sema &SemaRef);
   void WriteDeclUpdatesBlocks(RecordDataImpl &OffsetsRecord);
   void WriteDeclContextVisibleUpdate(const DeclContext *DC);
   void WriteFPPragmaOptions(const FPOptionsOverride &Opts);