[RuntimeAsync] PGO support in R2R for async methods

Currently we have the following TODO:

```c
// TODO: (async) PGO support for async variants
if (pMD->IsAsyncVariantMethod())
return false;
```

PGO is disabled for async callconv methods. While not critical, it would make sense to enable.

I am going to remove the TODO.
Let's use this issue to track the workitem.