[r2r] Include generic type instantiations from GenericLookupSignature for discovery of virtual methods

[BrzVlad]

Normally, we detect the usage of a type in the application via the TypeFixupSignature and then we use this type to determine which virtual methods on the type need compilation (via supporting GVMDependenciesNode and VirtualMethodUseNode).

Consider the following scenario:

public class TestA<T, U>
{
    public virtual void TestMethod(T item) => Console.WriteLine($"{item} / {typeof(U)}");
}

public class TestB<T, U>
{
    public TestA<T, U> TestCreate() => new TestA<T, U>();
}

TestB<string, int> obj = new TestB<string, int>();
obj.TestCreate().TestMethod("hello");

When compiling TestB<Canon,int32>.TestCreate we need to create an instance of TestA<T,int32>. Because we are compiling a shared version we need to embed a call to a generic lookup helper in order to obtain the actual T. This is done with a GenericLookupSignature fixup, which contains the type TestA<Canon,int32> that needs resolving. When creating this node, we add the InheritedVirtualMethodsNode as a dependency which facilitates discovery of virtual methods on this type.

Fixes perf on linq sorting, ex: LinqBenchmarks.Order00LinqMethodX (~20x improvement)

[Copilot]

Pull request overview

This PR updates ReadyToRun dependency analysis so that when a type handle is obtained via a GenericLookupSignature (generic dictionary lookup), the compiler still records the type usage needed to discover and compile inherited virtual methods for that instantiated type. It also adds a focused regression test covering the scenario.

Changes:

• Add InheritedVirtualMethods dependency tracking for GenericLookupSignature when _fixupKind == ReadyToRunFixupKind.TypeHandle.
• Add a new VirtualMethodGenerics test case that reaches a generic type instantiation only via a GenericLookupSignature fixup.
• Extend R2RTestSuites with a new [Fact] that validates the relevant virtual method is compiled.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/GenericLookupSignature.cs	Adds dependency analysis to record generic-lookup type usage for virtual dispatch discovery (mirrors TypeFixupSignature’s behavior for this scenario).
src/coreclr/tools/aot/ILCompiler.ReadyToRun.Tests/TestCases/VirtualMethodGenerics/GenericLookup.cs	New embedded test-case source driving a generic instantiation through a GenericLookupSignature-only path.
src/coreclr/tools/aot/ILCompiler.ReadyToRun.Tests/TestCases/R2RTestSuites.cs	Adds an xUnit test validating TestA\2<__Canon,int>.TestMethod` is compiled as expected.