[clang-tools-extra] [include-cleaner][NFC] expose and test `normalizePath` helper (PR #189364)

[Daniil Dudkin via cfe-commits]

unterumarmung wrote:

I marked it NFC because I do not think that `/` can actually be passed in the `include-cleaner`'s `normalizePath`, so it is not user-facing bugfix.

https://github.com/llvm/llvm-project/pull/189364