Lazify Target::llvm_target field

[Urgau]

This PR lazify the Target::llvm_target field by introducing MaybeLazy, a 3-way lazy container (borrowed, owned and lazied state).

Split from #122703
r? @petrochenkov

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[Urgau]

@bors try @rust-timer queue

[bors]

⌛ Trying commit b997b6a with merge 10163ec...

[bors]

☀️ Try build successful - checks-actions
Build commit: 10163ec (10163ecd4fb5702ed6496df4bd5a5f0c811262fd)

[rust-timer]

Finished benchmarking commit (10163ec): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

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

Instruction count

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

Max RSS (memory usage)

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

Cycles

Results (secondary -1.4%)

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)	1.9%	[1.9%, 1.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-3.2%, -2.9%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 690.539s -> 691.447s (0.13%)
Artifact size: 323.81 MiB -> 323.89 MiB (0.02%)

[petrochenkov]

No perf benefits + increased complexity => better not to do this.
We may reconsider when const eval becomes good enough to build target specs at compile time.