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

[Daniel Chen via cfe-commits]

DanielCChen 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.

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