[clang] [clang][driver] Prevent clang picking the system headers when started via a symlink (PR #68091)

[Liviu Ionescu via cfe-commits]

ilg-ul wrote:

> ... I understand this is only a bug on MacOS?

I'm using clang only to compile my binary packages on macOS (the Linux binaries are compiled with gcc), so I got bitten by this bug only on macOS. The other platforms/targets may or may not be affected, I don't know; but my initial search for InstalledDir showed multiple references not paired with references to Dir.

https://github.com/llvm/llvm-project/pull/68091