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

[Jon Roelofs via cfe-commits]

jroelofs wrote:

> Can you clearly state why you are against this PR? You don't consider the current behaviour of silently using the wrong headers a bug, or the proposed solution is not correct?

>> Don't consider this an objection

I am not against it; I just wanted to talk through all the options.

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