Update ILVerify to honor the "async" flag

[jcouv]

Runtime specification: https://github.com/dotnet/runtime/blob/main/docs/design/specs/runtime-async.md

Note: ilasm/ildasm were already updated to recognize the "async" keyword

Relates to test plan dotnet/roslyn#75960

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[Copilot]

Pull Request Overview

This PR adds support for async method verification in ILVerification. The changes enable the verifier to properly handle async methods that return Task or ValueTask types by unwrapping their return types and validating the stack state accordingly.

Key changes:

• Modified return statement verification logic to detect async methods and unwrap Task/ValueTask return types
• Added helper method to identify and unwrap Task, ValueTask, Task, and ValueTask types
• Added comprehensive test coverage for async method verification scenarios

Reviewed Changes

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

File	Description
src/tests/ilverify/ILTests/RuntimeAsyncTests.ilproj	Project file for new async method verification tests
src/tests/ilverify/ILTests/RuntimeAsyncTests.il	IL test cases covering valid and invalid async method patterns
src/coreclr/tools/ILVerification/ILImporter.Verify.cs	Implementation of async method return type validation logic

[jakobbotsch]

/ba-g Timeouts

[jakobbotsch]

I am not sure if ILVerify tests are running in outerloop. I don't see an "ilverify" work item, and when I follow the instructions in the readme I do not get any ILVerificationTests.cmd file. @jkoritzinsky Fall out from test merging? Any idea how to run these tests now?

[jakobbotsch]

I think Jeremy is OOF. I was able to validate the tests locally so I think this is good to merge. I opened #121594 about the testing.