Update to NRefactory commit b880fc60.

b880fc6 Fix possible NullReferenceException in XmlDocIssue.GatherVisitor.CheckXmlDoc()
9f6b757 Fix potential NullReferenceException in RedundantExtendsListEntryIssue
f73c7c0 Fix NullReferenceException in RedundantThisQualifierIssue.
3d2fb77 Fix possible ArgumentNullException in CS0169FieldIsNeverUsedIssue and related issues.
e8e5240 Fix ArgumentOutOfRangeException in ResolveVisitor.VisitArrayCreateExpression().
3082317 Fix NullReferenceException in AddArgumentNameAction.
b92f505 Fix NullReferenceException in NullAnalysisVisitor.VisitObjectCreateExpression
72d62f6 Guard ikvm loader against null namespaces.
ef6b76e Fix NullReferenceException in OwnedParameterReference.Resolve.
dcaa3a2 Fix constant folding for binary operators on enum values if mscorlib isn't loaded.
2c8646d Suppress ExplicitConversionInForEachIssue if variable has unknown type.
1ae6ba2 Fixed bug in CopyCommentsFromInterface
9deefa8 Fixed potiental crash in ConvertImplicitToExplicitImplementationAction
c5ca3ac Fixed add imports bug.
721ce12 Filter out synthetic types in import items.
909088b Ignored failing unit test.
75c6ad3 AlignEmbeddedIf/Using statement is now AlignEmbeddedStatements & works now with embedded lock statements as well.