Add test for runtime async and update linker handling of method body

[jtschuster]

Fixes #120340

Adds test with some rutnime async methods, and fixes the check for invalid IL when a Task returning async method doesn't have a value on the stack for ret.

[Copilot]

Pull Request Overview

This PR adds support for runtime async methods in the linker's dataflow analysis and test infrastructure. It fixes an issue where the linker would incorrectly validate IL for runtime async methods that return Task but don't have a value on the stack for the ret instruction.

Key changes:

• Adds test infrastructure support for /features and /nowarn compiler arguments
• Creates comprehensive test cases for runtime async methods with various dataflow scenarios
• Updates method body scanner to handle runtime async methods correctly

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
TestCaseCompiler.cs	Adds support for /features and /nowarn compiler arguments in test compilation
RuntimeAsyncMethods.cs	New test file with comprehensive coverage of runtime async method dataflow scenarios
MethodDefinitionExtensions.cs	Adds IsRuntimeAsync() extension method to detect runtime async methods
MethodBodyScanner.cs	Updates IL validation logic to handle runtime async methods that don't have return values on stack

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[agocke]

LGTM, thanks!

[sbomer]

LGTM, thank you!