Update Cargo.toml to use LLVM-12

[rennergade]

Minor update to use newer LLVM version

[CLAassistant]

All committers have signed the CLA.

[woodruffw]

The CI also probably needs to be updated to install LLVM 12 instead (it probably passes coincidentally at the moment, because of a default installation). Would you mind doing that as well?

[rennergade]

Sure. I updated the versions. Is there a way to test this implicitly?

[smoelius]

Sure. I updated the versions. Is there a way to test this implicitly?

I think the policy we should adopt is: the LLVM version to which siderophile is pinned should be the same as that of the latest stable rust.

$ rustc --version --verbose
rustc 1.54.0 (a178d0322 2021-07-26)
binary: rustc
commit-hash: a178d0322ce20e33eac124758e837cbd80a6f633
commit-date: 2021-07-26
host: x86_64-apple-darwin
release: 1.54.0
LLVM version: 12.0.1
              ^^^^^^

siderophile insists on using the default compiler. siderophile should check that the default compiler uses the LLVM version it expects. And, as a consequence, CI should fail when there is a mismatch.

I will open a separate issue for this, and we can continue the discussion there.

[smoelius]

We will also have to update the README (here), but I can do that after we merge.

@woodruffw I am going to merge this and close #145 if this looks good to you.

[smoelius]

Thanks a lot, @rennergade.