[llvm] [memprof] Drop memprof:: in unit tests (NFC) (PR #119113)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> lgtm but I still see a lot of other llvm::memprof:: prefixes - can the tests just have a using llvm::memprof and remove the rest too?

Yeah, `using llvm::memprof` sounds good.  Let me follow up.

https://github.com/llvm/llvm-project/pull/119113