New analyzer: Do not call BeginInvoke on a delegate for .NET Core

[sharwell]

This would apply to projects targeting .NET Standard or .NET Core.

https://github.com/dotnet/corefx/issues/5940 dotnet/runtime#16312
dotnet/winforms#1548

[danmoseley]

Or EndInvoke, right?

[sharwell]

@danmoseley I wouldn't expect users to continue calling EndInvoke after the calls to BeginInvoke were removed. Isn't the return value from BeginInvoke one of the arguments to EndInvoke?

[LunarWhisper]

@danmoseley I wouldn't expect users to continue calling EndInvoke after the calls to BeginInvoke were removed. Isn't the return value from BeginInvoke one of the arguments to EndInvoke?

Task implements IAsyncResult so we need to check EndInvoke too.