[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
```

Author: aaupov

bolt/test/perf2bolt/perf_test.test

@@ -4,12 +4,12 @@ REQUIRES: system-linux, perf
 
 RUN: %clang %S/Inputs/perf_test.c -fuse-ld=lld -Wl,--script=%S/Inputs/perf_test.lds -o %t
 RUN: perf record -Fmax -e cycles:u -o %t2 -- %t
-RUN: perf2bolt %t -p=%t2 -o %t3 -nl -ignore-build-id 2>&1 | FileCheck %s
+RUN: perf2bolt %t -p=%t2 -o %t3 -nl -ignore-build-id --show-density 2>&1 | FileCheck %s
 
 CHECK-NOT: PERF2BOLT-ERROR
 CHECK-NOT: !! WARNING !! This high mismatch ratio indicates the input binary is probably not the same binary used during profiling collection.
 CHECK: BOLT-INFO: Functions with density >= {{.*}} account for 99.00% total sample counts.
 
 RUN: %clang %S/Inputs/perf_test.c -no-pie -fuse-ld=lld -o %t4
 RUN: perf record -Fmax -e cycles:u -o %t5 -- %t4
-RUN: perf2bolt %t4 -p=%t5 -o %t6 -nl -ignore-build-id 2>&1 | FileCheck %s
+RUN: perf2bolt %t4 -p=%t5 -o %t6 -nl -ignore-build-id --show-density 2>&1 | FileCheck %s