[llvm] [llvm][ADT] Add PointerUnion benchmarks. NFC. (PR #187874)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `llvm` at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/45376

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
...
PASS: Flang :: Transforms/OpenACC/acc-implicit-copy-reduction.fir (4305 of 4315)
PASS: Flang :: Semantics/modfile73.f90 (4306 of 4315)
PASS: Flang :: Transforms/OpenACC/acc-implicit-data-fortran.F90 (4307 of 4315)
PASS: Flang :: Semantics/modfile72.f90 (4308 of 4315)
PASS: Flang :: Transforms/debug-dwarf-version.fir (4309 of 4315)
PASS: Flang :: Driver/use-module.f90 (4310 of 4315)
PASS: Flang :: Lower/OpenMP/rtl-flags.f90 (4311 of 4315)
PASS: Flang :: Transforms/vscale-attr.fir (4312 of 4315)
PASS: Flang :: Driver/linker-options.f90 (4313 of 4315)
PASS: Flang :: Driver/omp-driver-offload.f90 (4314 of 4315)
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2287.036944

```

</details>

https://github.com/llvm/llvm-project/pull/187874