[clang] [Driver] Enable LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON on AIX. (PR #132821)

[Louis Dionne via cfe-commits]

https://github.com/ldionne commented:

I think this should also impact the set of headers being searched. On per-target runtimes, I think we look for libc++ headers under `include/c++/v1/<target>` in addition to `include/c++/v1` (since that's where `__config_site` ends up)? I'm not super familiar with that configuration since we don't use it yet on Apple platforms, but I think it's worth checking that you're doing the right thing here.

https://github.com/llvm/llvm-project/pull/132821