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

[Fangrui Song via cfe-commits]

MaskRay wrote:

Ping:)

@ldionne I know you are discussing a `-isysroot` related issue (don't recall which one off my head) for Apple platforms. I think unifying InstalledDir and Dir will help.

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