[clang] [Darwin][Driver][clang] Prioritise `-isysroot` over `--sysroot` consistently (PR #115993)

[Cyndy Ishida via cfe-commits]

cyndyishida wrote:

 > The main concern is it happens when setting `DEFAULT_SYSROOT`. If only `isysroot` is supported on macOS, does this mean `DEFAULT_SYSROOT` should be deprecated on Apple platforms?

I don't think it's deprecated in the sense we have plans to drop support for it. The problem here seems more like when those values conflict what should be expected?

Today it seems expected that `sysroot` will be used for library search but not header search, which also matches the other platforms. I don't know if this adds confusion or helps unblock, but clang on Darwin also respects the env var `SDKROOT` which translates to `isysroot`. 

https://github.com/llvm/llvm-project/pull/115993