Add runtime-async varargs testcase and treat vararg Task-returning methods as normal methods

[Copilot]

__arglist (vararg calling convention) is an unsupported combination with runtime-async. Previously an async MethodImpl vararg method could produce a broken async thunk (crash/UB), and there was no coverage for vararg Task/ValueTask methods. The goal is "don't crash on scenarios that used to work."

Runtime change (src/coreclr/vm/methodtablebuilder.cpp)

In EnumerateClassMethods, before async-variant creation, classify vararg Task/ValueTask-returning methods deterministically:

• Ordinary (non-async) vararg Task/ValueTask method → re-classified as NormalMethod, so no async variant is generated and it behaves exactly as before.
• async MethodImpl vararg method → throws IDS_EE_VARARG_NOT_SUPPORTED (TypeLoadException) at type load instead of emitting a broken thunk.

// Vararg methods are not allowed to be marked MethodImpl.Async
// and even when they return Tasks they are still treated as NormalMethod.
if (returnKind != MethodReturnKind::NormalMethod)
{
    if (MetaSig::IsVarArg(Signature(pMemberSignature, cMemberSignature)))
    {
        if (IsMiAsync(dwImplFlags))
            BuildMethodTableThrowException(IDS_EE_VARARG_NOT_SUPPORTED);
        else
            returnKind = MethodReturnKind::NormalMethod;
    }
}

Test (src/tests/async/varargs/varargs.il + varargs.ilproj)

Authored in IL because Roslyn rejects __arglist in async methods under runtime-async (CS9328).

• Ordinary vararg Task/ValueTask methods (using ArgIterator): validated via direct call and via await from a runtime-async wrapper.
• async vararg Task/ValueTask methods: asserting the declaring type fails to load with TypeLoadException.
• Vararg call sites are isolated in dedicated helpers and each Fact is guarded with OperatingSystem.IsWindows(), since managed varargs are Windows-only (compFeatureVarArg, src/coreclr/jit/target.h); non-Windows platforms skip without jitting a vararg instruction.

Validation notes

The positive vararg paths cannot be exercised on Linux (managed __arglist is unsupported off-Windows — the JIT emits BADCODE), hence the Windows runtime gating; those paths depend on Windows CI for full validation. The runtime fix compiles cleanly.

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