VSTHRD200 should consider derived type of awaitable type as awaitable

[r-ramesh]

Derived types of awaitable types are not considered awaitable:

https://github.com/microsoft/vs-threading/blob/00eb19d63f25b7da7f6b950bf277def5858fcd37/src/Microsoft.VisualStudio.Threading.Analyzers/DiagnosticAnalyzerState.cs#L35-L49

This results in false alarm of VSTHRD200 if the method name ends in Async but returns a derived type of awaitable type.