Auto merge of rust-lang#118770 - saethlin:fix-inline-never-uses, r=nnethercote

Fix cases where std accidentally relied on inline(never)

This PR increases the power of `-Zcross-crate-inline-threshold=always` so that it applies through `#[inline(never)]`. Note that though this is called "cross-crate-inlining" in this case especially it is _just_ lazy per-CGU codegen. The MIR inliner and LLVM still respect the attribute as much as they ever have.

Trying to bootstrap with the new `-Zcross-crate-inline-threshold=always` change revealed two bugs:

We have special intrinsics `assert_inhabited`, `assert_zero_valid`, and `assert_mem_uniniitalized_valid` which codegen backends will lower to nothing or a call to `panic_nounwind`.  Since we may not have any call to `panic_nounwind` in MIR but emit one anyway, we need to specially tell `MirUsedCollector` about this situation.

`#[lang = "start"]` is special-cased already so that `MirUsedCollector` will collect it, but then when we make it cross-crate-inlinable it is only assigned to a CGU based on whether `MirUsedCollector` saw a call to it, which of course we didn't.

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I started looking into this because rust-lang#118683 revealed a case where we were accidentally relying on a function being `#[inline(never)]`, and cranking up cross-crate-inlinability seems like a way to find other situations like that.

r? `@nnethercote` because I don't like what I'm doing to the CGU partitioning code here but I can't come up with something much better

Author: bors

std/src/rt.rs

@@ -155,7 +155,6 @@ fn lang_start_internal(
 }
 
 #[cfg(not(test))]
-#[inline(never)]
 #[lang = "start"]
 fn lang_start<T: crate::process::Termination + 'static>(
     main: fn() -> T,