Revert "[clang] Match -isysroot behaviour with system compiler on Darwin"

This reverts commit f24aa691.

This change caused these two test failures on Darwin CI:
Clang.Tooling.clang-check-mac-libcxx-abspath.cpp
Clang.Tooling.clang-check-mac-libcxx-relpath.cpp

https://green.lab.llvm.org/green/job/clang-stage1-cmake-RA-incremental/37169/

More info in https://reviews.llvm.org/D157283