Auto merge of rust-lang#3480 - RalfJung:alloc_error_handler, r=RalfJung

directly call handle_alloc_error

Also test more codepaths. There's like 5 different things that can happen on allocation failure! Between `-Zoom`, `#[alloc_error_handler]`, and `set_alloc_error_hook`, we have 3 layers of behavior overrides. It's all a bit messy.

rust-lang/rust#112331 seems intended to clean this up, but has not yet reached consensus.

Author: bors