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

[Bo Anderson via cfe-commits]

Bo98 wrote:

> The expected flow on Apple platforms is to only pass an `isysroot` argument whether it's inherited from `xcrun -sdk <sdk version> clang` or passed explicitly. Could homebrew instead only pass `isysroot` for Darwin targets? Or check that `sysroot` and `isysroot` inputs are the same when creating an invocation. It's unclear to me how homebrew gets into a situation where there are conflicting sdks passed for `sysroot` and `isysroot` when you effectively want to ignore whatever is passed to `sysroot`.

In the past it was as simple as it happens when setting `DEFAULT_SYSROOT`. If only `isysroot` is supported on macOS, does this mean `DEFAULT_SYSROOT` should be deprecated on Apple platforms?

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