Auto merge of #134326 - scottmcm:slice-drop-shim-ptrmetadata, r=saethlin

Use `PtrMetadata` instead of `Len` in slice drop shims

I tried to do a bigger change in #134297 which didn't work, so here's the part I really wanted: Removing another use of `Len`, in favour of `PtrMetadata`.

Split into two commits where the first just adds a test, so you can look at the second commit to see how the drop shim for an array changes with this PR.

Reusing the same reviewer from the last one:
r? BoxyUwU

Author: bors