Re-apply "[Parse] Split incremental-extensions" (#66446)

Re-applies #65683 with a fix to always run
`Actions.ActOnEndOfTranslationUnit` regardless of incremental
processing.

Author: bnbarham

clang/include/clang/Lex/Preprocessor.h

@@ -277,6 +277,9 @@ class Preprocessor {
   /// Empty line handler.
   EmptylineHandler *Emptyline = nullptr;
 
+  /// True to avoid tearing down the lexer etc on EOF
+  bool IncrementalProcessing = false;
+
 public:
   /// The kind of translation unit we are processing.
   const TranslationUnitKind TUKind;
@@ -1910,14 +1913,11 @@ class Preprocessor {
   void recomputeCurLexerKind();
 
   /// Returns true if incremental processing is enabled
-  bool isIncrementalProcessingEnabled() const {
-    return getLangOpts().IncrementalExtensions;
-  }
+  bool isIncrementalProcessingEnabled() const { return IncrementalProcessing; }
 
   /// Enables the incremental processing
   void enableIncrementalProcessing(bool value = true) {
-    // FIXME: Drop this interface.
-    const_cast<LangOptions &>(getLangOpts()).IncrementalExtensions = value;
+    IncrementalProcessing = value;
   }
 
   /// Specify the point at which code-completion will be performed.

clang/lib/Lex/PPLexerChange.cpp

@@ -541,7 +541,7 @@ bool Preprocessor::HandleEndOfFile(Token &Result, bool isEndOfMacro) {
   Result.startToken();
   CurLexer->BufferPtr = EndPos;
 
-  if (isIncrementalProcessingEnabled()) {
+  if (getLangOpts().IncrementalExtensions) {
     CurLexer->FormTokenWithChars(Result, EndPos, tok::annot_repl_input_end);
     Result.setAnnotationEndLoc(Result.getLocation());
     Result.setAnnotationValue(nullptr);

clang/lib/Lex/Preprocessor.cpp

@@ -146,6 +146,10 @@ Preprocessor::Preprocessor(std::shared_ptr<PreprocessorOptions> PPOpts,
     Ident_AbnormalTermination = nullptr;
   }
 
+  // Default incremental processing to -fincremental-extensions, clients can
+  // override with `enableIncrementalProcessing` if desired.
+  IncrementalProcessing = LangOpts.IncrementalExtensions;
+
   // If using a PCH where a #pragma hdrstop is expected, start skipping tokens.
   if (usingPCHWithPragmaHdrStop())
     SkippingUntilPragmaHdrStop = true;

clang/lib/Parse/Parser.cpp

@@ -615,6 +615,11 @@ bool Parser::ParseTopLevelDecl(DeclGroupPtrTy &Result,
                                Sema::ModuleImportState &ImportState) {
   DestroyTemplateIdAnnotationsRAIIObj CleanupRAII(*this);
 
+  // Skip over the EOF token, flagging end of previous input for incremental
+  // processing
+  if (PP.isIncrementalProcessingEnabled() && Tok.is(tok::eof))
+    ConsumeToken();
+
   Result = nullptr;
   switch (Tok.getKind()) {
   case tok::annot_pragma_unused:
@@ -1038,7 +1043,7 @@ Parser::ParseExternalDeclaration(ParsedAttributes &Attrs,
       ConsumeToken();
       return nullptr;
     }
-    if (PP.isIncrementalProcessingEnabled() &&
+    if (getLangOpts().IncrementalExtensions &&
         !isDeclarationStatement(/*DisambiguatingWithExpression=*/true))
       return ParseTopLevelStmtDecl();