[Clang] [NFC] Remove default argument in ASTUnit.h (#78566)

This removes a default argument that is currently broken in C++23 mode
due to `std::default_delete` now being `constexpr`. This is a known
problem (see #74963, #59966, #69996, and a couple more), fixing which
will probably take some time, so this at least makes it possible to
compile `ASTUnit.h` in C++23 mode.

Note that we can’t simply include the header that provides the
definition of the class causing the problem either, as that would create
a circular dependency.

clang/include/clang/Frontend/ASTUnit.h

@@ -902,7 +902,7 @@ class ASTUnit {
                     SourceManager &SourceMgr, FileManager &FileMgr,
                     SmallVectorImpl<StoredDiagnostic> &StoredDiagnostics,
                     SmallVectorImpl<const llvm::MemoryBuffer *> &OwnedBuffers,
-                    std::unique_ptr<SyntaxOnlyAction> Act = nullptr);
+                    std::unique_ptr<SyntaxOnlyAction> Act);
 
   /// Save this translation unit to a file with the given name.
   ///

clang/tools/libclang/CIndexCodeCompletion.cpp

@@ -765,7 +765,8 @@ clang_codeCompleteAt_Impl(CXTranslationUnit TU, const char *complete_filename,
                     IncludeBriefComments, Capture,
                     CXXIdx->getPCHContainerOperations(), *Results->Diag,
                     Results->LangOpts, *Results->SourceMgr, *Results->FileMgr,
-                    Results->Diagnostics, Results->TemporaryBuffers);
+                    Results->Diagnostics, Results->TemporaryBuffers,
+                    /*SyntaxOnlyAction=*/nullptr);
 
   Results->DiagnosticsWrappers.resize(Results->Diagnostics.size());