Use a probe to avoid registering stray region obligations when re-checking drops in MIR typeck
Fixes#137288.
See the comment I left on the probe. I'm not totally sure why this depends on *both* an unconstrained type parameter in the impl and a type error for the self type, but I think the fix is at least theoretically well motivated.
r? ```@matthewjasper```
3a04ec8c56