Rollup merge of #121196 - Nilstrieb:the-clever-solution, r=saethlin

Always inline check in `assert_unsafe_precondition` with cfg(debug_assertions)

The current complexities in `assert_unsafe_precondition` are delicately balancing several concerns, among them compile times for the cases where there are no debug assertions. This comes at a large runtime cost when the assertions are enabled, making the debug assertion compiler a lot slower, which is very annoying.

To avoid this, we always inline the check when building with debug assertions.

Numbers (compiling stage1 library after touching core):
- master: 80s
- just adding `#[inline(always)]` to the `cfg(bootstrap)` `debug_assertions` (equivalent to a bootstrap bump (uhh, i just realized that i was on a slightly outdated master so this bump might have happened already), (#121112)): 67s
- this: 54s

So this seems like a good solution. I think we can still get the same run-time perf improvements for other users too by massaging this code further (see my other PR about adding `#[rustc_no_mir_inline]` #121114) but this is a simpler step that solves the imminent problem of "holy shit my rustc is sooo slow".

Funny consequence: This now means compiling the standard library with dbeug assertions makes it faster (than without, when using debug assertions downstream)!

r? ```@saethlin``` (or anyone else if someone wants to review this)

fixes #121110, supposedly

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