Auto merge of #119878 - scottmcm:inline-always-unwrap, r=workingjubilee

Tune the inlinability of `unwrap`

Fixes #115463
cc `@thomcc`

This tweaks `unwrap` on ~~`Option` &~~ `Result` to be two parts:
- `#[inline(always)]` for checking the discriminant
- `#[cold]` for actually panicking

The idea here is that checking the discriminant on a `Result` ~~or `Option`~~ should always be trivial enough to be worth inlining, even in `opt-level=z`, especially compared to passing it to a function.

As seen in the issue and codegen test, this will hopefully help particularly for things like `.try_into().unwrap()`s that are actually infallible, but in a way that's only visible with the inlining.

EDIT: I've restricted this to `Result` to avoid combining effects

Author: bors