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

[Liviu Ionescu via cfe-commits]

ilg-ul wrote:

> Only one of `Dir`/`InstalledDir` should be used

The current behaviour is:

- if `InstalledDir/../include/c++/v1` exists, it is used
- otherwise the `MacOSX.sdk/usr/include/c++/v1` is used.

In most cases, when invoked via a link, there is no `include` in that folder (see the example above), and the sdk folder is wrongly used.

The proposed behaviour is:

- if `InstalledDir/../include/c++/v1` exists, it is used
- if `Dir/../include/c++/v1` exists, it is used
- otherwise the `MacOSX.sdk/usr/include/c++/v1` is used.

Only one of them is used.

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