[BOLT][test] Fix perf_test

The interaction between `--show-density` and NFC testing wrapper is a
bit subtle. ShowDensity is enabled by default in perf2bolt mode, which
is detected based on executable name. NFC wrapper however replaces
the invocation of perf2bolt with `llvm-bolt --aggregate-only`, and so
ShowDensity is not enabled, which caused NFC test to fail:
https://lab.llvm.org/buildbot/#/builders/92/builds/18656

Add show-density flag explicitly to work around the issue.

Test Plan:
```
../llvm-project/bolt/utils/nfc-check-setup.py
bin/llvm-lit -a tools/bolt/test/perf2bolt/perf_test.test
```
1 file changed
tree: 6b918fc47a06fd6ab04c74c8911ddbb4513bb9ed
  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
README.md

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