Revert "[Parse] Split incremental-extensions" (#66281)

This reverts commit c2fb112, which
breaks:
```
lldb-api.commands/expression/diagnostics.TestExprDiagnostics.py
lldb-api.lang/objc/modules.TestObjCModules.py
lldb-api.lang/objc/modules-incomplete.TestIncompleteModules.py
lldb-api.lang/objc/modules-non-objc-target.TestObjCModulesNonObjCTarget.py
lldb-api.lang/objc/modules-objc-property.TestModulesObjCProperty.py
```

clang/include/clang/Lex/Preprocessor.h

@@ -277,9 +277,6 @@ 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;
@@ -1913,11 +1910,14 @@ class Preprocessor {
   void recomputeCurLexerKind();
 
   /// Returns true if incremental processing is enabled
-  bool isIncrementalProcessingEnabled() const { return IncrementalProcessing; }
+  bool isIncrementalProcessingEnabled() const {
+    return getLangOpts().IncrementalExtensions;
+  }
 
   /// Enables the incremental processing
   void enableIncrementalProcessing(bool value = true) {
-    IncrementalProcessing = value;
+    // FIXME: Drop this interface.
+    const_cast<LangOptions &>(getLangOpts()).IncrementalExtensions = 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 (getLangOpts().IncrementalExtensions) {
+  if (isIncrementalProcessingEnabled()) {
     CurLexer->FormTokenWithChars(Result, EndPos, tok::annot_repl_input_end);
     Result.setAnnotationEndLoc(Result.getLocation());
     Result.setAnnotationValue(nullptr);

clang/lib/Lex/Preprocessor.cpp

@@ -146,10 +146,6 @@ 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

@@ -615,11 +615,6 @@ 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:
@@ -711,8 +706,7 @@ bool Parser::ParseTopLevelDecl(DeclGroupPtrTy &Result,
 
     // Late template parsing can begin.
     Actions.SetLateTemplateParser(LateTemplateParserCallback, nullptr, this);
-    if (!PP.isIncrementalProcessingEnabled())
-      Actions.ActOnEndOfTranslationUnit();
+    Actions.ActOnEndOfTranslationUnit();
     //else don't tell Sema that we ended parsing: more input might come.
     return true;
 
@@ -1044,7 +1038,7 @@ Parser::ParseExternalDeclaration(ParsedAttributes &Attrs,
       ConsumeToken();
       return nullptr;
     }
-    if (getLangOpts().IncrementalExtensions &&
+    if (PP.isIncrementalProcessingEnabled() &&
         !isDeclarationStatement(/*DisambiguatingWithExpression=*/true))
       return ParseTopLevelStmtDecl();