Invalid cast from BuiltinFunctionInfo to BuiltinClassInfo in GetBuiltinType

Josh Smeaton (@jarshwah)

From #391:

[Error - 5:37:55 PM] System.InvalidCastException: Unable to cast object of type 'Microsoft.PythonTools.Analysis.Values.BuiltinFunctionInfo' to type 'Microsoft.PythonTools.Analysis.Values.BuiltinClassInfo'.
   at Microsoft.PythonTools.Analysis.PythonAnalyzer.GetBuiltinType(IPythonType type)
   at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext()
   at System.Collections.Generic.List`1.AddEnumerable(IEnumerable`1 enumerable)
   at System.Collections.Generic.List`1.InsertRange(Int32 index, IEnumerable`1 collection)
   at Microsoft.PythonTools.Analysis.Values.Mro..ctor(IEnumerable`1 values)
   at Microsoft.PythonTools.Analysis.Values.BuiltinClassInfo.get_Mro()
   at Microsoft.PythonTools.Analysis.Values.Mro.Recompute()
   at Microsoft.PythonTools.Analysis.Values.ClassInfo.SetBases(IEnumerable`1 bases)
   at Microsoft.PythonTools.Analysis.ClassAnalysisUnit.AnalyzeWorker(DDG ddg, CancellationToken cancel)
   at Microsoft.PythonTools.Analysis.Analyzer.DDG.Analyze(Deque`1 queue, CancellationToken cancel, Action`1 reportQueueSize, Int32 reportQueueInterval)
   at Microsoft.PythonTools.Analysis.PythonAnalyzer.AnalyzeQueuedEntries(CancellationToken cancel)
   at Microsoft.PythonTools.Intellisense.AnalysisQueue.GroupAnalysis.Analyze(CancellationToken cancel)
   at Microsoft.PythonTools.Intellisense.AnalysisQueue.HandleAnalyzable(IAnalyzable item, AnalysisPriority priority, CancellationToken cancellationToken)
   at Microsoft.PythonTools.Intellisense.AnalysisQueue.ConsumerLoop()

#439 changed a bunch of the MRO stuff, and that PR's timing lines up with when Josh Smeaton (@jarshwah) saw this.

It seems to me like either:

We now add more things to the MRO, which the (older) GetBuiltinType method needs to better handle.
The check in GetBuiltinType is correct, and the new code adds things it shouldn't to the MRO.