Fix flaky Driver tests
Several Driver tests create a hard link to the compiler in a temporary directory, then invoke it thorugh that hard link to see how it locates items in the resource directory. This pattern can tickle a system-load-dependent macOS bug involving invocations of freshly-created hard links, causing rare test failures in CI or on contributors’ machines.
This change avoids the OS bug by always copying instead of hard linking. We already fall back to copying on Windows, so all tests should pass with a copy anyway. Removing this workaround will be tracked by rdar://problem/53507844.
