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

[Fangrui Song via cfe-commits]

MaskRay wrote:

I am unfamiliar with Darwin and @jansvoboda11 @ldionne can give a more authoritative answer, but I can share some feelings.

* Gnu.cpp uses `Generic_GCC::addLibCxxIncludePaths` so other versions (e.g. `usr/include/c++/v2`) are supported.
* Only one of `Dir`/`InstalledDir` should be used. If they are overridden by `-ccc-install-dir dir`, `path(clang)/../include/c++` should  be ignored.

This patch does not move toward the ideal situation IMHO.

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