Rollup merge of #92992 - kornelski:backtraceopt, r=Mark-Simulacrum

Rollup merge of #92992 - kornelski:backtraceopt, r=Mark-Simulacrum

Help optimize out backtraces when disabled

The comment in `rust_backtrace_env` says:

>    // If the `backtrace` feature of this crate isn't enabled quickly return
>   // `None` so this can be constant propagated all over the place to turn
>  // optimize away callers.

but this optimization has regressed, because the only caller of this function had an alternative path that unconditionally (and pointlessly) asked for a full backtrace, so the disabled state couldn't propagate.

I've added a getter for the full format that respects the feature flag, so that the caller will now be able to really optimize out the disabled backtrace path. I've also made `rust_backtrace_env` trivially inlineable when backtraces are disabled.