roslyn: VB Overrides completion is very slow (~10s) for subtypes of InternalSyntax.VisualBasicSyntaxVisitor

1. In Compilers\VisualBasic\Test\Syntax\Parser\ParseXml.vb, add the following declaration within the ParseXml type:

Private Class Visitor : Inherits InternalSyntax.VisualBasicSyntaxVisitor
End Class

1. In the body of the new type, type Overrides and hit Ctrl-Space to bring up Completion.

It takes roughly 10 seconds (on a fast machine) for the dialog to appear.

It looks like roughly half of the CPU time is spent trying to compute the minimally-qualified name. Presumably, this is because every name in the InternalSyntax namespace conflicts with a name in the parent namespace (also in scope).