Rollup merge of #144011 - Zalathar:check-compiler-no-llvm, r=Kobzol

bootstrap: Don't trigger an unnecessary LLVM build from check builds

Coming back to r-l/r development after a few weeks away, I found a major regression in my dev workflows: running `x check compiler` (either manually or via rust-analyzer) would have the side-effect of building LLVM, even though that shouldn't be necessary.

For my main build directory this would be a minor annoyance, but for my separate rust-analyzer build directory it's a huge problem because it causes a completely separate build of LLVM, which takes a long time and should be completely unnecessary.

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After some investigation, I tracked down the problem to the `can_skip_build` check in this code:

https://github.com/rust-lang/rust/blob/3014e79f9c8d5510ea7b3a3b70d171d0948b1e96/src/bootstrap/src/core/build_steps/compile.rs#L1382-L1396

Historically, this would skip the LLVM build for stage 0 check builds. But after the recent stage 0 std redesign and some associated check stage renumbering (e.g. rust-lang/rust#143048), the condition `builder.top_stage == build_stage` is now false, because `top_stage` is 1 (due to the renumbering) but `build_stage` is 0 (because a “stage 1” non-library check build still uses the stage 0 compiler).

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Because this is a critical contributor roadblock for me, I have tried to fix this in a relatively narrow way. It's possible that all of this surrounding logic could be greatly simplified (especially in light of the stage redesign changes), but I didn't want this fix to be held back by scope creep.

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(Zulip thread: https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Bootstrap.20incorrectly.20building.20LLVM.20for.20check.20builds/near/528991035)

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