`compiletest::runtest`: `aggressive_rm_rf` does not like symlinks on Windows

[jieyouxu]

While reviewing #128562 and testing I discovered that runtest.rs's aggressive_rm_rf is not aggressive enough, unfortunately, because symlink-to-dir and symlink-to-file on Windows need different handling.

Have not considered junction points and hard links.

This can cause a rmake.rs test that creates symlinks to have its output artifacts fail to be removed on non-fresh test runs.