[clang][serialization] Enable ASTWriter to work with Preprocessor only (llvm#115237)

This PR builds on top of
llvm#115235 and makes it possible
to call `ASTWriter::WriteAST()` with `Preprocessor` only instead of full
`Sema` object. So far, there are no clients that leverage the new
capability - that will come in a follow-up commit.

Author: jansvoboda11

clang/include/clang/Serialization/ASTWriter.h

@@ -564,7 +564,7 @@ class ASTWriter : public ASTDeserializationListener,
   void WriteHeaderSearch(const HeaderSearch &HS);
   void WritePreprocessorDetail(PreprocessingRecord &PPRec,
                                uint64_t MacroOffsetsBase);
-  void WriteSubmodules(Module *WritingModule, ASTContext &Context);
+  void WriteSubmodules(Module *WritingModule, ASTContext *Context);
 
   void WritePragmaDiagnosticMappings(const DiagnosticsEngine &Diag,
                                      bool isModule);
@@ -585,7 +585,7 @@ class ASTWriter : public ASTDeserializationListener,
   void WriteComments(ASTContext &Context);
   void WriteSelectors(Sema &SemaRef);
   void WriteReferencedSelectorsPool(Sema &SemaRef);
-  void WriteIdentifierTable(Preprocessor &PP, IdentifierResolver &IdResolver,
+  void WriteIdentifierTable(Preprocessor &PP, IdentifierResolver *IdResolver,
                             bool IsModule);
   void WriteDeclAndTypes(ASTContext &Context);
   void PrepareWritingSpecialDecls(Sema &SemaRef);
@@ -642,7 +642,7 @@ class ASTWriter : public ASTDeserializationListener,
   void WriteDeclAbbrevs();
   void WriteDecl(ASTContext &Context, Decl *D);
 
-  ASTFileSignature WriteASTCore(Sema &SemaRef, StringRef isysroot,
+  ASTFileSignature WriteASTCore(Sema *SemaPtr, StringRef isysroot,
                                 Module *WritingModule);
 
 public:
@@ -662,10 +662,13 @@ class ASTWriter : public ASTDeserializationListener,
   /// include timestamps in the output file.
   time_t getTimestampForOutput(const FileEntry *E) const;
 
-  /// Write a precompiled header for the given semantic analysis.
+  /// Write a precompiled header or a module with the AST produced by the
+  /// \c Sema object, or a dependency scanner module with the preprocessor state
+  /// produced by the \c Preprocessor object.
   ///
-  /// \param SemaRef a reference to the semantic analysis object that processed
-  /// the AST to be written into the precompiled header.
+  /// \param Subject The \c Sema object that processed the AST to be written, or
+  /// in the case of a dependency scanner module the \c Preprocessor that holds
+  /// the state.
   ///
   /// \param WritingModule The module that we are writing. If null, we are
   /// writing a precompiled header.
@@ -676,8 +679,9 @@ class ASTWriter : public ASTDeserializationListener,
   ///
   /// \return the module signature, which eventually will be a hash of
   /// the module but currently is merely a random 32-bit number.
-  ASTFileSignature WriteAST(Sema &SemaRef, StringRef OutputFile,
-                            Module *WritingModule, StringRef isysroot,
+  ASTFileSignature WriteAST(llvm::PointerUnion<Sema *, Preprocessor *> Subject,
+                            StringRef OutputFile, Module *WritingModule,
+                            StringRef isysroot,
                             bool ShouldCacheASTInMemory = false);
 
   /// Emit a token.

clang/lib/Frontend/ASTUnit.cpp

@@ -2359,7 +2359,7 @@ bool ASTUnit::Save(StringRef File) {
 
 static bool serializeUnit(ASTWriter &Writer, SmallVectorImpl<char> &Buffer,
                           Sema &S, raw_ostream &OS) {
-  Writer.WriteAST(S, std::string(), nullptr, "");
+  Writer.WriteAST(&S, std::string(), nullptr, "");
 
   // Write the generated bitstream to "Out".
   if (!Buffer.empty())