[Clang][Parse][NFC] Use llvm::function_ref<> instead of std::optional<llvm::function_ref<>> (#142906)

There is no need to distinguish between null `optional` and null
`function_ref` in this case.

Author: zwuis

clang/include/clang/Parse/Parser.h

@@ -2598,8 +2598,7 @@ class Parser : public CodeCompletionHandler {
   void ParseTypeQualifierListOpt(
       DeclSpec &DS, unsigned AttrReqs = AR_AllAttributesParsed,
       bool AtomicOrPtrauthAllowed = true, bool IdentifierRequired = false,
-      std::optional<llvm::function_ref<void()>> CodeCompletionHandler =
-          std::nullopt);
+      llvm::function_ref<void()> CodeCompletionHandler = {});
 
   /// ParseDirectDeclarator
   /// \verbatim

clang/lib/Parse/ParseDecl.cpp

@@ -6124,8 +6124,7 @@ bool Parser::isConstructorDeclarator(bool IsUnqualified, bool DeductionGuide,
 
 void Parser::ParseTypeQualifierListOpt(
     DeclSpec &DS, unsigned AttrReqs, bool AtomicOrPtrauthAllowed,
-    bool IdentifierRequired,
-    std::optional<llvm::function_ref<void()>> CodeCompletionHandler) {
+    bool IdentifierRequired, llvm::function_ref<void()> CodeCompletionHandler) {
   if ((AttrReqs & AR_CXX11AttributesParsed) &&
       isAllowedCXX11AttributeSpecifier()) {
     ParsedAttributes Attrs(AttrFactory);
@@ -6145,7 +6144,7 @@ void Parser::ParseTypeQualifierListOpt(
     case tok::code_completion:
       cutOffParsing();
       if (CodeCompletionHandler)
-        (*CodeCompletionHandler)();
+        CodeCompletionHandler();
       else
         Actions.CodeCompletion().CodeCompleteTypeQualifiers(DS);
       return;
@@ -7236,9 +7235,9 @@ void Parser::ParseFunctionDeclarator(Declarator &D,
       ParseTypeQualifierListOpt(
           DS, AR_NoAttributesParsed,
           /*AtomicOrPtrauthAllowed=*/false,
-          /*IdentifierRequired=*/false, llvm::function_ref<void()>([&]() {
+          /*IdentifierRequired=*/false, [&]() {
             Actions.CodeCompletion().CodeCompleteFunctionQualifiers(DS, D);
-          }));
+          });
       if (!DS.getSourceRange().getEnd().isInvalid()) {
         EndLoc = DS.getSourceRange().getEnd();
       }

clang/lib/Parse/ParseDeclCXX.cpp

@@ -2649,9 +2649,9 @@ void Parser::MaybeParseAndDiagnoseDeclSpecAfterCXX11VirtSpecifierSeq(
   // handled by the caller.  Diagnose everything else.
   ParseTypeQualifierListOpt(
       DS, AR_NoAttributesParsed, /*AtomicOrPtrauthAllowed=*/false,
-      /*IdentifierRequired=*/false, llvm::function_ref<void()>([&]() {
+      /*IdentifierRequired=*/false, [&]() {
         Actions.CodeCompletion().CodeCompleteFunctionQualifiers(DS, D, &VS);
-      }));
+      });
   D.ExtendWithDeclSpec(DS);
 
   if (D.isFunctionDeclarator()) {