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

[Ben Langmuir via cfe-commits]

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@@ -4858,14 +4860,21 @@ time_t ASTWriter::getTimestampForOutput(const FileEntry *E) const {
   return IncludeTimestamps ? E->getModificationTime() : 0;
 }
 
-ASTFileSignature ASTWriter::WriteAST(Sema &SemaRef, StringRef OutputFile,
-                                     Module *WritingModule, StringRef isysroot,
-                                     bool ShouldCacheASTInMemory) {
+ASTFileSignature
+ASTWriter::WriteAST(llvm::PointerUnion<Sema *, Preprocessor *> Subject,
+                    StringRef OutputFile, Module *WritingModule,
+                    StringRef isysroot, bool ShouldCacheASTInMemory) {
   llvm::TimeTraceScope scope("WriteAST", OutputFile);
   WritingAST = true;
 
-  ASTHasCompilerErrors =
-      SemaRef.PP.getDiagnostics().hasUncompilableErrorOccurred();
+  Sema *SemaPtr = Subject.dyn_cast<Sema *>();
+  Preprocessor &PPRef = [&]() -> Preprocessor & {
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benlangmuir wrote:

Is there a reason not to use a ternary expression here? 

https://github.com/llvm/llvm-project/pull/115237