EventPipe: Stacks for sampling events are being discarded

[noahfalk]

Running the stack sampling provider produces events that have no stacks. This appears to be a recent regression between Preview5 and Preview6 and I tracked it to this line:

https://github.com/dotnet/coreclr/blob/master/src/vm/eventpipesession.cpp#L311

The line should read:

m_pBufferManager->WriteEvent(pThread, *this, event, payload, pActivityId, pRelatedActivityId, pEventThread, pStack)

cc @dotnet/dotnet-diag

[noahfalk]

I tested the fix in my change (#24853), but looking at the code deltas I think @jorive's change fixed this independently in his earlier PR dotnet/coreclr#24896.