[llvm] [NFC][LLVM] Namespace cleanup in LegacyPassManagerTest (PR #196811)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM.

Although too much cleanup in the LegacyPM probably isn't necessary. It should just get deleted eventually.

https://github.com/llvm/llvm-project/pull/196811