Auto merge of #135384 - saethlin:inline-copy-from-slice, r=joboet

Add #[inline] to copy_from_slice

I'm doing cooked things to CGU partitioning for compiler-builtins (rust-lang/rust#135395) and this was the lone symbol in my compiler-builtins rlib that wasn't an intrinsic. Adding `#[inline]` makes it go away.

Perf report indicates a marginal but chaotic effect on compile time, marginal improvement in codegen. As expected.

Author: bors