[clang-tools-extra] [Clangd][NFC] Normalize all file path root names (PR #153736)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

Making this change in code specific to the "rename" operation is just going to address one symptom of a larger underlying problem, which is https://github.com/clangd/clangd/issues/108.

A more comprehensive fix for the underlying issue is being worked on in https://github.com/llvm/llvm-project/pull/136439.

https://github.com/llvm/llvm-project/pull/153736