[clang] Add more tests for _Countof (PR #133333)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thanks for the additional test coverage! It looks like precommit CI found some issues with your tests that should be addressed: https://buildkite.com/llvm-project/github-pull-requests/builds/162502#0195d9e8-2c29-426e-9be0-afda7efd7d93

https://github.com/llvm/llvm-project/pull/133333