[clang] [clang] Match -isysroot behaviour with system compiler on Darwin (PR #80524)

[Daniel Rodríguez Troitiño via cfe-commits]

drodriguez wrote:

@ilg-ul: It seems that the driver does not receive any `-isysroot`, but it seems that `cc1` receives `-isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk`, so I am suspicious of `SDKROOT` maybe being in the environment. Can you check if your environment is providing that value under the hood?

https://github.com/llvm/llvm-project/pull/80524