Assorted bootstrap LLVM refactors (part 2/N)

[Kobzol]

Continuation of #160645.

This PR removes dependence on implicit LLVM CI paths, and generally tries to make depending on various LLVM things more explicit wherever possible. One nice outcome of that is that we no longer override the LLVM config during config parsing when using download-ci-llvm 🎉 We also no longer dynamically download LLVM from CI when someone asks what is the LLVM linkage mode, lol (well, we still need to do it in a few places, but now at least it is explicit).

I tried to ensure that x check compiler does not build or even checkout the LLVM submodule. That being said, there might be some cases where calling get_llvm_build_status might now checkout the LLVM submodule (but it shouldn't happen for x test mir-opt --bless, which was fixed in #130918 - that PR introduced a boolean flag, but it was difficult to understand where it should be set to false/true).

The next thing after this is to try getting rid of the llvm_out function, though that will be probably quite painful. And also explicitly depending on the output of LlvmFromCi, rather than reading llvm_ci_mode.

I printed some step traces using commands like this:

BOOTSTRAP_TRACING=STEP=trace BOOTSTRAP_TRACING_SKIP_TIME=1 ./x build compiler 2> post-cg_default-build-compiler.txt

with this PR and on main.

The BOOTSTRAP_TRACING_SKIP_TIME thing was added in the first commit to make it easier to compare step traces. But later I realized that we should also normalize source code locations of the steps, otherwise if you modify something in a given file with the step, its location will change and it will perturb the diff. Anyway, I can drop the first commit if you want.

Best reviewed commit-by-commit (in fact I don't think it can be really reviewed in another way, lol).

r? jieyouxu

[rustbot]

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.