Update NRefactory to commit 1663c9a.

1663c9a Fix NamespaceDefinitionOverwritesTypeName unit test.
b6845ed Fix icsharpcode/NRefactory#385 - Can't resolve indexer with protected access modifier
16f49e7 Try to fix a namespace name lookup bug.
5aded3f Fixed bug in 'ConvertToLambdaExpressionIssue'.
61c10b1 Categorized some code issues.
2b54760 Fixed bug in 'DelegateSubtractionIssue'.
bae55fb Fixed 'IsInactiveConditionalMethod' in SemanticHighlightingVisitor.
2a7a8bb Fixed possible infinite loop.
8acf7df Fix CSharpAmbience output for classes in the global namespace.
830e7d6 Fixed bug in xmldoc issue.
df27cc6 Revert "Merged Hint & Suggestion severity. (These two are too close and"
fc6f5c7 Merged Hint & Suggestion severity. (These two are too close and doesn't really make sense to separate)
7eaf69e Fixed 'Bug 17945 - Bad 'unnecessary cast' warning'.
008b2b0 Fix potential NullReferenceException in TypeSystemAstBuilder.ConvertConstructor
b92cbff Fix potential InvalidCastException in AccessToClosureIssue.
5a31546 Fix NullReferenceException when calling GetBaseMembers() on global method.
b31436e Merge pull request icsharpcode/NRefactory#384 from DavidKarlas/master
93ab8b3 Fixing Constructor Initializer placement and new adding policies NewLineBeforeConstructorInitializerColon and NewLineAfterConstructorInitializerColon
a219cff Fixed a bug in constant value type system conversion.
5056b18 Fixed unit tests.
a31d543 Improved GetTypeForQueryVariable
f5d76b8 Removed resolveinvocationongiventarget call.
0dcf0a3 Added another work around for the query variable issue.
cdd1892 Revert "Try to use a syntactical approach for resolving lambda expressions."
ffdb464 Try to use a syntactical approach for resolving lambda expressions.
1bd7434 Added failing unit test.
be4d1c0 Improved CS0618UsageOfObsoleteMemberIssue
d20744e Set XmlResolver=null to avoid looking up external DTDs.
ff9fee2 Fix bug in AstNode.GetNodesBetween().