[clang-repl] Fix error recovery while PTU cleanup (#127467)

Fixes #123300

What is seen 
```
clang-repl> int x = 42;
clang-repl> auto capture = [&]() { return x * 2; };
In file included from <<< inputs >>>:1:
input_line_4:1:17: error: non-local lambda expression cannot have a capture-default
    1 | auto capture = [&]() { return x * 2; };
      |                 ^
zsh: segmentation fault  clang-repl --Xcc="-v"

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x8)
  * frame #0: 0x0000000107b4f8b8 libclang-cpp.19.1.dylib`clang::IncrementalParser::CleanUpPTU(clang::PartialTranslationUnit&) + 988
    frame #1: 0x0000000107b4f1b4 libclang-cpp.19.1.dylib`clang::IncrementalParser::ParseOrWrapTopLevelDecl() + 416
    frame #2: 0x0000000107b4fb94 libclang-cpp.19.1.dylib`clang::IncrementalParser::Parse(llvm::StringRef) + 612
    frame #3: 0x0000000107b52fec libclang-cpp.19.1.dylib`clang::Interpreter::ParseAndExecute(llvm::StringRef, clang::Value*) + 180
    frame #4: 0x0000000100003498 clang-repl`main + 3560
    frame #5: 0x000000018d39a0e0 dyld`start + 2360
```

Though the error is justified, we shouldn't be interested in exiting
through a segfault in such cases.

The issue is that empty named decls weren't being taken care of
resulting into this assert


https://github.com/llvm/llvm-project/blob/c1a229252617ed58f943bf3f4698bd8204ee0f04/clang/include/clang/AST/DeclarationName.h#L503

Can also be seen when the example is attempted through xeus-cpp-lite.


![image](https://github.com/user-attachments/assets/9b0e6ead-138e-4b06-9ad9-fcb9f8d5bf6e)
2 files changed
tree: 23731cbd5941c954a9b2ecdff428fd91fc394706
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
  20. mlir/
  21. offload/
  22. openmp/
  23. polly/
  24. pstl/
  25. runtimes/
  26. third-party/
  27. utils/
  28. .clang-format
  29. .clang-format-ignore
  30. .clang-tidy
  31. .git-blame-ignore-revs
  32. .gitattributes
  33. .gitignore
  34. .mailmap
  35. CODE_OF_CONDUCT.md
  36. CONTRIBUTING.md
  37. LICENSE.TXT
  38. pyproject.toml
  39. README.md
  40. SECURITY.md
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