[clang] [Driver] Unify InstalledDir and Dir (PR #80527)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

>From my point of view, I think this sounds fine - it looks like you've looked through this and thought deeper about it than I have at least. I'm not deep enough into this to comfortably press approved on this for now though, so hopefully someone else can chime in as well.

https://github.com/llvm/llvm-project/pull/80527