Stop using LinkedGraph in lexical_region_resolve

[Zalathar]

There are only two users of the older LinkedGraph data structure, and this is one.

It turns out that this diagnostic-related code doesn't need any non-trivial graph operations (since it does its own graph traversal); it just needs the ability to get a list of in-edges or out-edges (constraints) for any particular node. That's easy enough to do with a simple custom data structure.

Inspired by #152621, which wants to make changes to LinkedGraph that wouldn't make sense for this use-site.

[rustbot]

r? @fmease

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[Zalathar]

I don't expect this to be on a hot path for perf, but let's check.

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 71438fe (71438fe7132483fe73da8fa49bc361feb85ab6dd, parent: d3e8bd94cc042c65e30a6c60146a8a00531d1292)

[rust-timer]

Finished benchmarking commit (71438fe): comparison URL.

Overall result: no relevant changes - 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

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

Max RSS (memory usage)

Results (primary 1.8%, secondary 2.4%)

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

	mean	range	count
Regressions ❌ (primary)	1.8%	[1.5%, 2.1%]	2
Regressions ❌ (secondary)	2.4%	[1.7%, 2.7%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.8%	[1.5%, 2.1%]	2

Cycles

Results (secondary -1.9%)

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

Binary size

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

Bootstrap: 480.206s -> 480.109s (-0.02%)
Artifact size: 397.91 MiB -> 397.93 MiB (0.00%)

[Zalathar]

@bors rollup=maybe