VSTHRD200 should consider derived type of awaitable type as awaitable

[r-ramesh]

Derived types of awaitable types are not considered awaitable:

vs-threading/src/Microsoft.VisualStudio.Threading.Analyzers/DiagnosticAnalyzerState.cs

Lines 35 to 49 in 00eb19d

	IMethodSymbol? getAwaiterMethod = typeSymbol.GetMembers(nameof(Task.GetAwaiter)).OfType<IMethodSymbol>().FirstOrDefault(m => m.Parameters.IsEmpty);
	if (getAwaiterMethod is object)
	{
	isAwaitable = ConformsToAwaiterPattern(getAwaiterMethod.ReturnType);
	}
	else
	{
	IEnumerable<ITypeSymbol>? awaitableTypesFromThisAssembly = from candidateAwaiterMethod in compilation.GetSymbolsWithName(m => m == GetAwaiterMethodName, SymbolFilter.Member, cancellationToken).OfType<IMethodSymbol>()
	where candidateAwaiterMethod.IsExtensionMethod && !candidateAwaiterMethod.Parameters.IsEmpty
	where ConformsToAwaiterPattern(candidateAwaiterMethod.ReturnType)
	select candidateAwaiterMethod.Parameters[0].Type;
	IEnumerable<ITypeSymbol>? awaitableTypesPerAssembly = from assembly in compilation.Assembly.Modules.First().ReferencedAssemblySymbols
	from awaitableType in GetAwaitableTypes(assembly)
	select awaitableType;
	isAwaitable = awaitableTypesFromThisAssembly.Concat(awaitableTypesPerAssembly).Contains(typeSymbol);

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