Update LLVM

[bdrewery]

r? @alexcrichton

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

Ah turns out we've already got another LLVM update in flight, so I've added this to that PR and that'll hopefully land as smoothly as this would have otherwise have done so!

[bdrewery]

Sounds good, thanks.