cl: stabilize generic local ABI names across cache loads

[cpunion]

Summary

• keep synthesized generic-local TypeName objects positionless because their generated names already encode the complete local and generic identity
• prevent ABI naming from appending a loader-relative .p<token.Pos> suffix that differs between cached archives and a later synthetic root module
• add a cross-process cold/hot package-cache regression and assert the frontend position invariant

This preserves the existing position fallback for genuine detached local-scope objects while making generic-local cache hits link reliably.

Fixes #2297

Testing

• go test ./cl -run "^TestGenericLocalTypePatchHelpers$" -count=1
• go test ./cl -run "^TestRunAndTestFromTestgo$" -count=1
• go test -tags=dev ./internal/build -run "^TestGenericLocalTypeColdAndHotPackageCache$" -count=1
• go test ./ssa/abi -count=1
• issue reproducer: forced cold llgo test -a -p=1 ... followed by the immediate cache-hit llgo test -p=1 ...; both pass

[fennoai]

Review summary

The fix is correct and well-targeted. Replacing t.Obj().Pos() with token.NoPos for the detached TypeName in patchLocalGenericNamed (cl/compile.go) removes the source of cross-process non-determinism: scopeIndices in ssa/abi/abi.go only appends the loader-relative .p<pos> suffix when pos.IsValid(), so token.NoPos drops that varying suffix while type identity is fully preserved by the type-arg-encoded localNamedName. The explanatory comment is accurate, and the subprocess-based cold/hot cache test is the right shape — an in-process test could not reproduce #2297 since the offending token.Pos comes from a per-process FileSet.

Security, documentation-accuracy, and performance passes surfaced no issues (token.NoPos is in fact a net reduction in per-type work). One minor, optional test-robustness finding is left inline.

No blocking issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

LLGo baseline benchmarks

2bcdbdafd654 | workflow run | long-term charts

Program measurements

Platform	Workload	File size	vs base	Build	vs base	Run	vs base
Linux	cprintf	18656 B	+0.0%	335.054 ms	+11.4% (worse)	1.410 ms	+2.8% (worse)
Linux	fmtprintf	1881664 B	+0.0%	3.216 s	+11.5% (worse)	4.344 ms	+2.1% (worse)
Linux	println	68512 B	+0.0%	317.873 ms	+8.6% (worse)	1.731 ms	+1.8% (worse)
macOS	cprintf	84672 B	+0.0%	374.691 ms	-40.6% (better)	3.151 ms	-66.6% (better)
macOS	fmtprintf	1889248 B	+0.0%	3.385 s	-35.5% (better)	13.616 ms	-29.1% (better)
macOS	println	121216 B	+0.0%	423.927 ms	-12.3% (better)	4.110 ms	-53.3% (better)

Core language and compiler benchmarks

Platform	Benchmark	ns/op	vs base
Linux	BenchmarkLookupPCRandom	12.270 ns/op	-0.5% (better)
Linux	BenchmarkMergeCompilerFlags	148.400 ns/op	+3.2% (worse)
Linux	BenchmarkMergeLinkerFlags	96.430 ns/op	+2.4% (worse)
Linux	BenchmarkChannelBuffered	36.310 ns/op	-0.0% (better)
Linux	BenchmarkChannelHandoff	25190 ns/op	-0.9% (better)
Linux	BenchmarkDefer	48.230 ns/op	-1.4% (better)
Linux	BenchmarkDirectCall	1.759 ns/op	+0.1% (worse)
Linux	BenchmarkGlobalRead	1.773 ns/op	+0.9% (worse)
Linux	BenchmarkGlobalWrite	2.807 ns/op	+0.1% (worse)
Linux	BenchmarkGoroutine	36084 ns/op	+17.5% (worse)
Linux	BenchmarkInterfaceCall	8.454 ns/op	+0.1% (worse)
Linux	BenchmarkRuntimeGetG	2.462 ns/op	+0.0%
macOS	BenchmarkLookupPCRandom	12.810 ns/op	+0.9% (worse)
macOS	BenchmarkMergeCompilerFlags	114.800 ns/op	-21.9% (better)
macOS	BenchmarkMergeLinkerFlags	82.540 ns/op	-7.9% (better)
macOS	BenchmarkChannelBuffered	27.770 ns/op	-34.1% (better)
macOS	BenchmarkChannelHandoff	7895 ns/op	-30.3% (better)
macOS	BenchmarkDefer	41.430 ns/op	-21.8% (better)
macOS	BenchmarkDirectCall	1.057 ns/op	-24.9% (better)
macOS	BenchmarkGlobalRead	1.123 ns/op	-37.4% (better)
macOS	BenchmarkGlobalWrite	1.125 ns/op	-55.1% (better)
macOS	BenchmarkGoroutine	46861 ns/op	-14.2% (better)
macOS	BenchmarkInterfaceCall	6.085 ns/op	-14.1% (better)
macOS	BenchmarkRuntimeGetG	2.420 ns/op	-29.9% (better)

Compared with c454c0e65b71 measured in the same runner job.