Auto merge of #100719 - CohenArthur:rust-safe-intrinsic-attribute, r=wesleywiser

Add `#[rustc_safe_intrinsic]`

This PR adds the `#[rustc_safe_intrinsic]` attribute as mentionned on Zulip. The goal of this attribute is to avoid keeping a list of symbols as the source for stable intrinsics, and instead rely on an attribute. This is similar to `#[rustc_const_stable]` and `#[rustc_const_unstable]`, which among other things, are used to mark the constness of intrinsic functions.

Author: bors