Fix UnreachableBody with only ref to assembly.

[mrvoorhe]

When an unreachable body contains a reference to another assembly, and there are no other references to that assembly, the linker would retain the reference but delete the assembly.

This was happening because the SweepStep was running before CodeRewriterStep. Which meant that when SweepStep.SweepAssemblyReferences iterated all of the types references in the assembly it would visit type references to method bodies that were going to be stubbed. By moving the CodeRewriterStep ahead of the sweep step we ensure that the SweepStep is visiting methods as they will look in the end.

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[Copilot]

Pull request overview

Fixes a linker pipeline ordering issue where SweepStep could retain an assembly reference originating only from method bodies that would later be stubbed by CodeRewriterStep, leading to inconsistent output (reference retained while the referenced assembly is deleted).

Changes:

• Reorders the standard linker pipeline to run CodeRewriterStep before SweepStep.
• Adds a new regression test covering the “only reference is inside an unreachable body” scenario.
• Adds a small dependency assembly used by the new test and updates the generated analyzer test list.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/tools/illink/src/linker/Linker/Driver.cs	Moves CodeRewriterStep ahead of SweepStep in the standard pipeline to ensure sweeping observes the final rewritten bodies.
src/tools/illink/test/Mono.Linker.Tests.Cases/UnreachableBody/OnlyReferenceInUnreachableBody.cs	New test validating that an assembly reference originating only in a stubbed body is removed (and the referenced assembly can be removed).
src/tools/illink/test/Mono.Linker.Tests.Cases/UnreachableBody/Dependencies/OtherAssembly2.cs	New dependency type compiled into other.dll for the new regression test.
src/tools/illink/test/ILLink.RoslynAnalyzer.Tests/generated/.../UnreachableBodyTests.g.cs	Adds the new test entry to the generated analyzer test suite.

[Copilot]

This test expects Foo.Method to be stubbed ([ExpectBodyModified]) and for the reference to other.dll to be removable. All other UnreachableBody tests explicitly enable the unreachablebodies optimization via [SetupLinkerArgument("--enable-opt", "unreachablebodies")]; without it, the body may not be rewritten and the test may not reproduce the scenario reliably. Add the --enable-opt unreachablebodies setup argument here as well.

Suggested change

	[SetupCompileBefore("other.dll", new[] { typeof(OtherAssembly2) })]
	[SetupCompileBefore("other.dll", new[] { typeof(OtherAssembly2) })]
	[SetupLinkerArgument("--enable-opt", "unreachablebodies")]

[Copilot]

CodeRewriterStep was previously executed after SweepStep, and there are existing comments/assumptions in the linker code that refer to that ordering (e.g., Linker.Steps/MarkStep.cs around the comment "CodeRewriterStep runs after sweeping"). With this pipeline reordering, please update any such references to avoid future confusion and keep the documentation consistent with the actual step order.

Suggested change

	p.AppendStep(new OutputWarningSuppressions());
	p.AppendStep(new OutputWarningSuppressions());
	// Code rewriting happens before sweeping in the current pipeline order.

[mrvoorhe]

@sbomer Do you have thoughts on this? Here is the code in the MarkStep

runtime/src/tools/illink/src/linker/Linker.Steps/MarkStep.cs

Line 3170 in 7c994a0

// CodeRewriterStep runs after sweeping, and may request the stubbed value for any preserved method

based on the comment it sounds intentional. I'm not that familiar with the FeatureGuardAttribute or FeatureSwitchDefinitionAttribute stuff that is mentioned in the comment.