There's no general way to set a pending managed exception in CoreCLR (the
current plan is to support setting a pending managed exception for the
objc_msgSend family of functions). This means that the way we've implemented
custom wrappers that can handle Objective-C exceptions won't work, because
those wrappers currently tries to set a pending managed exception (which Mono
throws upon returning from the corresponding native wrapper function).
So rewrite this a bit: these custom wrappers now return a GCHandle with the
managed exception as an out parameter, and the calling managed code throws
that exception instead.
This also required adjusting a few API definitions to match how their wrapper
functions are defined.
14e76c0139