Diagnostic for bad type arguments in cref could be more clear

The diagnostics introduced to fix 1087447 are confusing and deserve improvement. Specifically, for the source code

''' <summary>
''' <see cref="C(Of Integer).f()"/>
''' </summary>
Class C(Of T)
Sub f()
End Sub
End Class

we produce

BC42309: XML comment has a tag with a 'cref' attribute 'C(Of Integer).f()' that could not be resolved.
''' <see cref="C(Of Integer).f()"/>

The error is that the type argument isn't a simple identifier.

Migrated from TFS/DevDiv 1096332