Invalid cast from BuiltinFunctionInfo to BuiltinClassInfo in GetBuiltinType

[jakebailey]

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 @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.

[MikhailArkhipov]

Looks like function somehow leaked into SetBases. MRO should technically only contain classes so the direct cast fails.