Fix GVM dispatch for covariant-return async methods (closes #127197)

[Copilot]

ResolveGenericVirtualMethod was passing asyncVariantLookup = Ordinary (the default) to FindOrCreateAssociatedMethodDesc. When the vtable slot resolved to an async-variant or ReturnDroppingThunk (RDT) MethodDesc, GetParallelMethodDesc(pDefMD, Ordinary) walked back to the primary thunk instead. The primary thunk then emitted CALLVIRT back to the async-variant slot, which GVM dispatch again resolved to the primary thunk — infinite recursion → stack overflow.

The fix is a one-line change in ResolveGenericVirtualMethod:

// Before — asyncVariantLookup defaults to Ordinary, misidentifying async-variant/RDT targets
RETURN(MethodDesc::FindOrCreateAssociatedMethodDesc(
    pTargetMDBeforeGenericMethodArgs, pTargetMT,
    (pTargetMT->IsValueType()), pStaticMD->GetMethodInstantiation(),
    FALSE));

// After — propagate the async variant kind of the resolved vtable entry
RETURN(MethodDesc::FindOrCreateAssociatedMethodDesc(
    pTargetMDBeforeGenericMethodArgs, pTargetMT,
    (pTargetMT->IsValueType()), pStaticMD->GetMethodInstantiation(),
    FALSE,
    pTargetMDBeforeGenericMethodArgs->GetAsyncVariantLookup()));

GetAsyncVariantLookup() returns Async, ReturnDroppingThunk, or Ordinary depending on what the resolved slot holds, ensuring FindOrCreateAssociatedMethodDesc creates the correctly-typed instantiated MD in all three cases.

This fix depends on the prerequisite AsyncVariantLookup::ReturnDroppingThunk enum value, GetAsyncVariantLookup(), the 3-way variant selector in FindOrCreateAssociatedMethodDesc, and the bool → AsyncVariantLookup widening of InstMethodHashTable::FindMethodDesc / InstantiatedMethodDesc::FindLoadedInstantiatedMethodDesc already in this PR. TestGenericVirtualMethod is re-enabled.

Copilot generated this PR description. A human author reviewed and is responsible for these changes.

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[VSadov]

@copilot test failures need to be addressed

[Copilot]

@copilot test failures need to be addressed

The committed fix in ResolveGenericVirtualMethod (passing pTargetMDBeforeGenericMethodArgs->GetAsyncVariantLookup() to FindOrCreateAssociatedMethodDesc) is in the build that CI ran, but the test is still stack-overflowing at VirtualFunctionPointer → CallInstance.

After deep analysis of the dispatch chain:

1. CallInstance(new Derived()) → VirtualFunctionPointer(b, Base.$async_void<object>) → slot S → Mid.$rdt (inherited by Derived, since Derived doesn't introduce its own $rdt when fHasCovariantOverride is false for Derived)
2. The fix correctly returns Mid.InstanceMethod<object>$rdt_stub (instantiating stub wrapping Mid.InstanceMethod<__Canon>$rdt)
3. The canonical Mid.InstanceMethod<__Canon>$rdt calls VirtualFunctionPointer(this, Mid.$async_int<MVAR[0]>) via CALLVIRT in the thunk IL

The logic says this should resolve to Derived.InstanceMethod<object>$async_int without recursion. However, the test still overflows. I was not able to complete a local build to observe the actual behavior and determine the deeper root cause before running out of time. The fix needs further investigation by a human reviewer familiar with the runtime-async dispatch mechanics.