[clang] [clang][AIX] Handle triple environment component for per target runtime directory (PR #140850)

[Daniel Chen via cfe-commits]

DanielCChen wrote:

> A more general question:
> It seems neither `compile-rt` nor `flang-rt` uses `getTargetSubDirPath`, so we didn't need to strip to the "base" triple.
> Sanitizer uses it apparently. Is it by design?

The reason we didn't need to change `getTargetSubDirPath` is because we striped the triple in its caller, `getRuntimePath`. 
I think the best way is to do it in `getTargetSubDirPaht` instead of `getRuntimePath` as I suggested in my inline comment.

https://github.com/llvm/llvm-project/pull/140850