[beta] Bump LLVM to a upstream 12.0 release

[nagisa]

Currently the beta branch utilizes a release candidate of LLVM 12.0. Since then a large number of issues have been fixed upstream, including those that are known to affect Rust code.

It would be unfortunate if we released a stable Rust with all of those issues when we have the fixes in the actual LLVM release and/or our backports of commits that didn't make the release cut on top of it.

cc @cuviper / @nikic

[rust-highfive]

r? @cuviper

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[rust-highfive]

⚠️ Warning ⚠️

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!
• These commits modify submodules.

[cuviper]

#84230 is already beta-nominated -- shouldn't we wait for that process?

You also pulled in the stack-probe dwarf change here, not sure if that was intentional.

[nagisa]

Yes, it was intentional to pull in that change, though, now that I think about it, the DWARF problem is in stable already so we don't really need to backport it like we should with the others.

[nagisa]

Ah, I was wrong. while it would've reached stable, it did not because of #83412 and so it is valuable to consider this for backport, including separately from the other LLVM 12.0 backport.

[wesleywiser]

We discussed this in the compiler team triage meeting and decided to approve the backport.

[cuviper]

@bors r+

[bors]

📌 Commit 46a2c6a has been approved by cuviper

[bors]

⌛ Testing commit 46a2c6a with merge cc6d66f...

[bors]

☀️ Test successful - checks-actions
Approved by: cuviper
Pushing cc6d66f to beta...