translate saved current word span to the current snapshot in HighlightUsageTagger

*
SyntaxConstructClassifier unsubscribes from OnBuildProjConfigDoneEventHandler if doc.TextBuffer is null
remove redundant assertions
Dispose classifiers correctly
remove ITextBuffer from SyntaxConstructClassifierProvider dictionary after disposing
Merge pull request #479 from vasily-kirichenko/fix-wrong-current-word-snapshot

translate saved current word span to the current snapshot in HighlightUsageTagger