[clang] [clang][driver] Add \<executable\>/../include/c++/v1 to include path on Darwin (PR #70817)

[Louis Dionne via cfe-commits]

ldionne wrote:

Is there any reason why we don't just use `Dir` all the time? What's the difference between those two? I think I wrote this code a few years ago and I can guarantee you that I simply didn't know there was a difference between `Dir` and `InstalledDir` (since I still don't know the difference).

https://github.com/llvm/llvm-project/pull/70817