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

[Fangrui Song via cfe-commits]

MaskRay 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.

Thanks. Yes, I think should be done.

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Ping

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