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

[Daniel Chen via cfe-commits]

DanielCChen wrote:

> 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.

Thanks for the comment!
It doesn't seem there is any AIX specific configuration there. @daltenty Do you know if we need to do anything?

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