Rust: Re-enable LLVM and debug assertions for slow builders

[alexcrichton]

Disabled in a recent PR the builder is routinely hitting its 3 hour time limit, tracked at https://alexcrichton.github.io/rust-ci-timing-tracker/#series=x86_64-mingw

For a win in build times we're going to disable debug/llvm assertions on this builder, but this issue is intended to track re-enabling these if we ever get infrastructure that's a bit speedier or can better handle the slowdown.

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List of all the builders where assertions were disabled:

• x86_64-mingw
• x86_64-msvc
• i686-mingw-1
• i686-gnu
• i686-gnu-nopt
• i686-msvc-1
• i686-msvc-2
• x86_64-msvc-cargo

[alexcrichton]

For posterity, this had a pretty massive impact on CI times for the thre platforms that we disabled debug/llvm assertions for:

[aidanhs]

(currently waiting on a potential upgrade to our appveyor builder machines, maybe happening soonish)

[aidanhs]

Much has happened - we've migrated to azure in the meantime and our builds have got even slower, but there is potential to get 4 core machines there. Until then, this is still on hold.

[pietroalbini]

https://github.com/rust-lang/rust/pull/64517 also disabled LLVM assertions on the following builders:

• i686-gnu
• i686-gnu-nopt
• i686-msvc-1
• i686-msvc-2
• x86_64-msvc-cargo

[shepmaster]

Triage: Nothing has materially changed; we still don't have the time budget to re-enable these. Perhaps when the builders gain more cores.

[pietroalbini]

We'll probably want to re-enable those if we switch to GitHub Actions, as we have the time budget there.

[Mark-Simulacrum]

With GHA, we should now have capacity to do this on CI. I will be posting a PR doing so soon.