Tie extern "llvm-intrinsics" and llvm.* symbol names together

[bjorn3]

LLVM intrinsics must always use extern "llvm-intrinsic" and extern "llvm-intrinsic" must only be used for actual LLVM intrinsics.

Follow up to #161398

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[bjorn3]

Not sure if this is the best place to do this check.

[ShoyuVanilla]

Sorry, I'm pretty ignorant about this area 😅

r? compiler

[dingxiangfei2009]

I reckon that the patch is incomplete. Are you going to "sanitize" the extern "llvm-intrinsics" declarations so that we don't hit the assertions? I can wait a bit.

[bjorn3]

Both extern "llvm-intrinsic" and #[link_name = "llvm.*"] are unstable features marked as internal. As such I think it is fine to ICE whenever you abuse them per rust-lang/compiler-team#620. But if you really want I can change them to fatal errors.

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This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.