6307: Add whitelist of safe intrinsics r=frazar a=frazar
This PR should fix#5996, where intrinsic operations where all marked as unsafe.
I'm rather new to this codebase, so I might be doing something *very* wrong. Please forgive me!
In particular, I'm not sure how to "check that we are in extern `rust-intrinsics`" as mentioned [in this comment](https://github.com/rust-analyzer/rust-analyzer/issues/5996#issuecomment-709234802).
Co-authored-by: Francesco Zardi <frazar@users.noreply.github.com>
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