[llvm] [ADT] Make protected members of DenseMapBase private (NFC) (PR #228651)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> "Tighten member visibility " isn't very useful. Just say whether you make it private/protected. The description is lengthy - while the change is quite minimal.

Thanks for the review!  I've shortened the commit message.  Please take a look.  Thanks!

https://github.com/llvm/llvm-project/pull/228651