[llvm] [offload][nfc] Move AsyncInfoTy's definitions to libompaccsupport (PR #220991)

[Alex Duran via llvm-commits]

adurang wrote:

> LGTM, btw. I was wondering if we could remove this smaller wrapper in favor of `AsyncInfoWrapperTy` from `PluginInterface.h`? I see that libompaccsupport already depends on the header, so maybe we could remove the duplication? The classes are not the same and the one from `omptarget.h` has some things that are specific to openMP I think - `addPostProcessingFunction`, `getVoidPtrLocation`.

I don't think that's a good idea. It's bad enough that we're calling the Plugin interface directly. At least we shouldn't start using the internal data structures of the plugins.

https://github.com/llvm/llvm-project/pull/220991