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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> > changing the structure of the symlinks is not possible, it is done automatically by a tool.
> 
> To be fair: tools can be changed, and in fact you're proposing to change one ;)
> 
> But that said, now I see what you're up against.

I am more convinced that we should unify `InstalledDir` and `Dir`.
Right now include and library paths are partially derived from `Dir`/`ResourceDir` (derived from `ClangExecutable`) and partially derived from `InstalledDir`.

I believe that `InstalledDir` (2010 88979914d7e1666c4f32d41223cb614cdedf1f41) was a partial solution when `-no-canonical-prefixes` (2009) did not work well. `InstalledDir` should eventually be removed.

Created #80527

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