[libc++][hardening][NFC] Rename the `has-debug-mode` feature to `has-legacy-debug-mode`.

This will allow reusing the `has-debug-mode` feature for the new debug
mode without accidentally reenabling old debug mode tests. The
`has-legacy-debug-mode` feature is deliberately never set -- the old
tests are left checked in to have a point of reference for the level of
checking that was supported by the legacy debug mode, making it easier
to verify we've reached feature parity in the future.