R1:df21391b8e7b

When "apcu_clear_cache()" exists, prefer it as a cache clear callback over "apc_clear_cache()"

Summary:
See PHI36. APCu originally had `apc_` methods, but at some point dropped these and only provides `apcu_` methods.

When the `apcu_` method is present, use it. It may not be present for older versions of APCu, so keep the fallback.

Test Plan:
- With modern APCu, clicked "Purge Caches" in Config > Caches.
- Before: fatal on bad `apc_clear_caches` call.
- After: Valid cache clear.

Reviewers: chad…