Auto merge of #51041 - alexcrichton:better-unwind, r=nikomatsakis

std: Ensure OOM is classified as `nounwind`

OOM can't unwind today, and historically it's been optimized as if it can't
unwind. This accidentally regressed with recent changes to the OOM handler, so
this commit adds in a codegen test to assert that everything gets optimized away
after the OOM function is approrpiately classified as nounwind

Closes #50925