Resolve generic descriptor method signatures in ILC

[Copilot]

Description

NativeAOT could not resolve XML descriptor method signatures that reference generic parameters, e.g. System.Void Method(T), while ILLink accepted the same descriptor.

• Descriptor matching

  • Formats method signatures with method/type generic context.
  • Resolves signature variables back to their generic parameter names for XML comparison.

• ILTrim coverage and parity

  • Re-enables existing ILTrim coverage for descriptor-rooted methods by removing expected-failure entries that now pass.
  • Removes the test-specific --disable-opt unreachablebodies test workaround.
  • Adds an ILTrim IsWorthConvertingToThrow approximation so empty method bodies composed of an optional nop followed by ret are preserved as-is, matching ILLink behavior closely enough for these tests.
  • Uses that heuristic when deciding whether to rewrite unreachable method bodies.

• ILCompiler test coverage

  • Updates the shared LinkXml test expectation so NativeAOT/ILCompiler now expects the generic descriptor-rooted method to be kept.

Example descriptor now handled consistently:

<type fullname="MyGenericClass`1">
  <method signature="System.Void GenericMethod(T)" />
</type>

[MichalStrehovsky]

@copilot ILCompiler.Trimming.Tests are failing, I think it's because we actually fixed this for ILCompiler: Method Mono.Linker.Tests.Cases.LinkXml.UnusedMethodPreservedByLinkXmlIsKept.Unused.PreservedMethod5(T)' should have been removed`

[Copilot]

@copilot ILCompiler.Trimming.Tests are failing, I think it's because we actually fixed this for ILCompiler: Method Mono.Linker.Tests.Cases.LinkXml.UnusedMethodPreservedByLinkXmlIsKept.Unused...

Addressed in 5c81b46. Since ILCompiler now resolves the generic descriptor signature, I updated the shared test expectation so PreservedMethod5<T> is kept for NativeAOT too; ILCompiler.Trimming.Tests LinkXml tests now pass locally.

[Copilot]

Pull request overview

This PR improves descriptor-driven method signature matching so XML descriptors that reference generic parameters (e.g., System.Void GenericMethod(T)) resolve consistently across toolchains, and adjusts ILTrim’s unreachable-body rewriting behavior/tests to align more closely with ILLink/NativeAOT expectations.

Changes:

• Update XML descriptor signature formatting to include method/type generic context and print signature variables using their generic parameter names.
• Refine ILTrim unreachable-body rewriting to preserve extremely small bodies (ret and nop; ret) instead of converting them to a shared ldnull; throw stub.
• Re-enable/adjust test coverage by removing ILTrim expected-failure entries and updating LinkXml expectations.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/tools/illink/test/Mono.Linker.Tests.Cases/LinkXml/UnusedMethodPreservedByLinkXmlIsKept.cs	Updates test expectation so the descriptor-rooted generic method is expected to be kept generally.
src/coreclr/tools/ILTrim.Tests/ILTrimExpectedFailures.txt	Removes several entries that should now pass with the improved signature matching and unreachable-body behavior.
src/coreclr/tools/ILTrim.Core/DependencyAnalysis/TokenBased/MethodDefinitionNode.cs	Adds a small-body heuristic to avoid converting ret / nop; ret methods to an unreachable throw body when optimizing.
src/coreclr/tools/Common/Compiler/ProcessLinkerXmlBase.cs	Formats method signatures with generic context so signature variables can be emitted as generic parameter names for XML comparison.

[sbomer]

LGTM, thank you!

[MichalStrehovsky]

/ba-g the tests completed successfully, but there was some Python error after they were done. looks like infra hiccup