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

[Jon Roelofs via cfe-commits]

jroelofs wrote:

> In my use case, and probably in most use cases, the location where the link comes from is a plain folder, not a hierarchy specific to a toolchain.

I would have expected the opposite. Most non-windows clang toolchains that I know of typically ship `clang` and `clang++` as symlinks to a single binary named `clang-${version}`, all of which are located in the typical bin-next-to-lib toolchain folder structure.  Darwin toolchains are a _little_ weird, since we don't ship libc++ & co. in `bin/../{lib,include}`, and instead those go in an "SDK" (much like a "sysroot" on other platforms).

As an aside, there is also driver mode selection logic that depends heavily on the specific names of these symlinks, e.g. for C vs C++ mode, clang-cl mode, and/or cross-compiling to a specific target when prefixed with the target triple e.g. `arm-none-eabi-clang -> clang`.


Don't consider this an objection, but: for your use case, can you stick symlinks in to preserve that toolchain folder structure relative to the `~/tmp/clang++` symlink? i.e. make that `~/tmp/bin/clang++ -> /Users/ilg/Library/xPacks/@xpack-dev-tools/clang/15.0.7-3.1/.content/bin/clang-15` and add `~/tmp/lib -> /Users/ilg/Library/xPacks/@xpack-dev-tools/clang/15.0.7-3.1/.content/lib` (and same for `include`).

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