Fix dataflow analysis of lambdas in async methods

[MichalStrehovsky]

When we enter ILLinker part of the ILC codebase, we need to switch to the mode where async variants don't exist and async methods have mismatched bodies. There was a spot where we didn't do the switch here:

runtime/src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/UsageBasedMetadataManager.cs

Lines 939 to 949 in d31e599

	MethodIL methodILDefinition = methodIL.GetMethodILDefinition();
	if (FlowAnnotations.CompilerGeneratedState.TryGetUserMethodForCompilerGeneratedMember(methodILDefinition.OwningMethod, out var userMethod))
	{
	Debug.Assert(userMethod != methodILDefinition.OwningMethod);
	// It is possible that this will try to add the DatadlowAnalyzedMethod node multiple times for the same method
	// but that's OK since the node factory will only add actually one node.
	methodILDefinition = FlowAnnotations.ILProvider.GetMethodIL(userMethod);
	if (ShouldSkipDataflowForMethod(methodILDefinition))
	return;
	}

The fix is to do the switch early in FlowAnnotations so that the ILProvider FlowAnnotations use (and other parts take advantage of) is already ILLinkerified.

Fixes the situation shown in the regression test (we were analyzing the IL of the async variant of RuntimeAsyncWithLambda and missed analyzing the lambda completely).

Cc @dotnet/ilc-contrib

[dotnet-policy-service]

Tagging subscribers to this area: @agocke, @MichalStrehovsky, @jkotas
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR fixes a dataflow analysis bug where lambdas inside async methods were not being properly analyzed. The issue occurred because the async IL masking transformation wasn't applied consistently across all dataflow analysis components.

• Centralized AsyncMaskingILProvider wrapping in FlowAnnotations constructor
• Removed duplicate wrapping from TypeAnnotationsHashtable and MethodBodyScanner
• Added regression test for lambda analysis in async methods

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/Dataflow/FlowAnnotations.cs	Wraps the ILProvider with AsyncMaskingILProvider at construction time, ensuring all consumers use the masked version; removes duplicate wrapping from TypeAnnotationsHashtable
src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/Dataflow/MethodBodyScanner.cs	Removes duplicate AsyncMaskingILProvider wrapping since FlowAnnotations now provides a pre-wrapped ILProvider
src/tools/illink/test/Mono.Linker.Tests.Cases/DataFlow/RuntimeAsyncMethods.cs	Adds ExpectedNoWarnings attribute and new test case RuntimeAsyncWithLambda to verify lambdas in async methods are properly analyzed for dataflow warnings