clang: Make tests using symlinks more consistent.

Instead of checking if each symlink exists before removing it,
remove the whole temp dir housing the symlinks before recreating it.
This is a bit shorter, conceptually simpler (in that the first
and consecutive test runs have more similar behavior), it's what we're
already doing in almost all places where we do it, and it works if the
symlink exists but is a dead link (e.g. when it points into the build
dir but the build dir is renamed).

No intended behavior change.