[clang-tidy][NFC] add debug log when clean empty namespace

[HerrCai0907]

Summary

Adding debug log when clean empty namespace can make #60051 more cleaner.
User will not be confused why the whole namespace is deleted after fixing modernize-concat-nested-namespaces.

[PiotrZSL]

This shouldn't be delivered. Do such tests, locally, instead of committing them.
This change impact clang, and all tools, not only clang-tidy, therfor [clang-tidy] prefix is misleading.

Issue related to modernize-concat-nested-namespaces should be fixed by clang-tidy itself.
It's visible in AST, so clang-tidy could fix it.

TranslationUnitDecl
`-NamespaceDecl <line:1:1, line:3:1> line:1:11 a
  `-NamespaceDecl <line:2:1, col:14> col:11 b

For me this change should be abandoned.
And proper way of informing users would be to emit warning with that information from clang-tidy, not by adding some debug logs to code.

clang/lib/Format/Format.cpp

void checkEmptyNamespace(SmallVectorImpl<AnnotatedLine *> &AnnotatedLines) {
     for (unsigned i = 0, e = AnnotatedLines.size(); i != e; ++i) {
       auto &Line = *AnnotatedLines[i];
       if (Line.startsWithNamespace())
         checkEmptyNamespace(AnnotatedLines, i, i, DeletedLines);
     }
 
     for (auto Line : DeletedLines) {
       FormatToken *Tok = AnnotatedLines[Line]->First;
+      LLVM_DEBUG(llvm::dbgs() << "clean empty namespace:\"");
       while (Tok) {
+        LLVM_DEBUG(llvm::dbgs() << Tok->TokenText << " ");
         deleteToken(Tok);
         Tok = Tok->Next;
       }
+      LLVM_DEBUG(llvm::dbgs() << "\"\n");
     }
   }
 
   // The function checks if the namespace, which starts from \p CurrentLine, and
   // its nested namespaces are empty and delete them if they are empty. It also
   // sets \p NewLine to the last line checked.
   // Returns true if the current namespace is empty.
   bool checkEmptyNamespace(SmallVectorImpl<AnnotatedLine *> &AnnotatedLines,