ClangRefactoring: Improve symbol parsing

Declarations are only indexed if their file has been changed and references
has to be indexed if the file or any included file has been changed.

Change-Id: I07c6de1379bce2462c1e0fad34d4378a3da4397b
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangrefactoringbackend/source/symbolscollector.cpp

@@ -51,6 +51,7 @@ void SymbolsCollector::addUnsavedFiles(const V2::FileContainers &unsavedFiles)
 
 void SymbolsCollector::clear()
 {
+    m_indexDataConsumer->clear();
     m_collectMacrosSourceFileCallbacks.clear();
     m_symbolEntries.clear();
     m_sourceLocationEntries.clear();