Simplify CI LLVM checks in bootstrap

[Kobzol]

Extracted out of #138591. Apart from simplifying the checks, it will make it easier to run E2E tests of bootstrap on a mostly empty directory without checking out LLVM on CI :)

The commits should be mostly self-explanatory.

r? @onur-ozkan

[rustbot]

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

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.

[Kobzol]

Uhh, crap, this breaks the check-default-config-profiles.sh smoke test, because it tries to check each profile on CI, and the library profile uses download-ci-llvm...

[Kobzol]

Hmm, a lot of bootstrap tests parse flags to create a config, and the default value for download-ci-llvm is true. I'm not necessarily sure if that's a good default - shouldn't it be if-unchanged?

[onur-ozkan]

"if-unchanged" syncs llvm submodule, that's why true is the default value.

[Kobzol]

Rebased on top of #138743.

@rustbot blocked #138743

[onur-ozkan]

Changes look good but this is one of those PRs where it's hard to find issues without merging it... So let's try that now.

@bors r+ rollup=never

[bors]

📌 Commit f5c59a4 has been approved by onur-ozkan

It is now in the queue for this repository.

[bors]

⌛ Testing commit f5c59a4 with merge a4a11ac...

[bors]

☀️ Test successful - checks-actions
Approved by: onur-ozkan
Pushing a4a11ac to master...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 4ac032f (parent) -> a4a11ac (this PR)

Test differences

No test diffs found

[rust-timer]

Finished benchmarking commit (a4a11ac): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.1%	[-1.1%, -1.1%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary 3.7%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.7%	[3.5%, 4.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -0.8%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.8%	[-0.8%, -0.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.8%	[-0.8%, -0.8%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 775.474s -> 775.351s (-0.02%)
Artifact size: 365.53 MiB -> 365.55 MiB (0.00%)