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

[Daniel Rodríguez Troitiño via cfe-commits]

drodriguez wrote:

The other place that a default sysroot might come is the CMake option `DEFAULT_SYSROOT`. In my builds it is empty (the default), but it might be pointing to something in those xpack builds: https://github.com/xpack/xpack-build-box/blob/64488ebdfefd96e5eec45ab31bc170aa028fed4e/helper/common-apps-functions-source.sh#L9135

I think this is were that value is coming from. In all my testing that value is empty, so it is never a fallback, but it might have an actual value in your case, which makes `-isysroot` magically appear even if no value has been provided in the command line.

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