[clang] [Darwin][Driver][clang] Prioritise command line args over `DEFAULT_SYSROOT` (PR #115993)

[Carlo Cabrera via cfe-commits]

carlocab wrote:

> > Sorry, which env var are you referring to?
> 
> I'm suggesting an explicit test case that checks that `DEFAULT_SYSROOT` is set and behaves as expected. It's ignored if sysroot or isysroot is passed, it's used when they're missing.

`DEFAULT_SYSROOT` is not an environment variable. It is a CMake variable/macro that must be set at configure-time:

https://github.com/llvm/llvm-project/blob/aac000a01ba8c0ed15fec3c198fce9f13fc83725/clang/CMakeLists.txt#L208-L209

https://github.com/llvm/llvm-project/blob/aac000a01ba8c0ed15fec3c198fce9f13fc83725/clang/include/clang/Config/config.h.cmake#L48-L49

I don't think `lit` knows how to tell whether `DEFAULT_SYSROOT` is set. We could maybe teach it to, but given that that intent of #94284 is to deprecate it (and I think all usage of it in existing builders has been removed), that seems like the wrong direction.

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