Managing lifetimes of [UnmanagedFunctionPointer] delegates

[TChatzigiannakis]

As per the MSDN documentation, any delegate that's passed to unmanaged code as a function pointer needs to be explicitly kept alive in managed code, since the GC doesn't track pointers on the unmanaged side.

We have many such delegates in LLVMSharp. In LLVMSharp.Api, I intend to have the relevant wrapper classes manage their lifetimes so the user won't have to call GC.KeepAlive() on them. For example, a delegate that logically belongs to a Context will be kept alive privately and will be released only when the Context is Disposed.

[mjsabby]

Sounds good. Great work on the OOApi, I'm still focusing on getting this runnable on as many platforms, so it's good we're doing non overlapping stuff.