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
```
416c782cc6