[mlir][async] Allow to call async.execute inside async.func

[mlir][async] Allow to call async.execute inside async.func

This change added support of calling async execute inside async.func.
Ex.

```
async.func @async_func_call_func() -> !async.token {
  %token = async.execute {
    %c0 = arith.constant 0 : index
    memref.store %arg0, %arg1[%c0] : memref<1xf32>
    async.yield
  }
  async.await %token : !async.token
  return
}
```

Reviewed By: ezhulenev

Differential Revision: https://reviews.llvm.org/D141730