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

[David Tenty via cfe-commits]

daltenty wrote:

> @ldionne I have checked that on AIX, clang uses the configuration from the system include as `-internal-isystem /opt/IBM/openxlCSDK/include/c++/v1`. There is already a test in `aix-toolchain-include.cpp` for that.

Yeah, I don't think we implemented the resource directory based include path logic in the driver for AIX, though we should.

I think it makes sense to put that in a separate follow on PR, as that wasn't supported for either setting of `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` so it's not really specific to this change.

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