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

[James Y Knight via cfe-commits]

jyknight wrote:

Hm, I had a thought which might meet the needs here, and not add additional confusion: we could have the Darwin SDKROOT environment variable override both `-isysroot` and `--sysroot`. Would that help the original issue?

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