Split LTO out of the main codegen coordinator event loop into a separate event loop on the same thread

[bjorn3]

This will make it easier to in the future move all this code to link_binary.

Follow up to #146209
Part of rust-lang/compiler-team#908

[rustbot]

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

[rustbot]

r? @jackh726

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[bjorn3]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: c8901ae (c8901aea8472c485ddb8304c562f06f8b77a55ca, parent: a41214f9bd7ce869d21898f31107f92c90a9112f)

[rust-timer]

Finished benchmarking commit (c8901ae): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-1.0%, -0.4%]	7
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -1.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.8%	[0.8%, 0.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.1%	[-3.1%, -3.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.2%	[-3.1%, 0.8%]	2

Cycles

Results (secondary 2.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 476.516s -> 476.021s (-0.10%)
Artifact size: 390.37 MiB -> 390.39 MiB (0.01%)