[PGO] Add missing target datalayout in test (#169520)
The test was added by b8ef25aa643761233dc5b74d9fb7c38a2064d9c7. It
failed on at least the following bots, but the failure did not reproduce
on my test machines or in pre-commit CI:
- https://lab.llvm.org/buildbot/#/builders/190/builds/31638
- https://lab.llvm.org/buildbot/#/builders/190/builds/31638
This fix hopefully addresses at least the warnings there.
