Clang: Clear SymbolCollector before updating new project parts

Change-Id: I82e9bb4f66a7597bb911fbc97d6022ff3fdb0b35
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/tools/clangrefactoringbackend/source/collectsymbolsaction.h

@@ -63,6 +63,12 @@ public:
         return m_sourceLocationEntries;
     }
 
+    void clear()
+    {
+        m_symbolEntries.clear();
+        m_sourceLocationEntries.clear();
+    }
+
 private:
     SymbolEntries m_symbolEntries;
     SourceLocationEntries m_sourceLocationEntries;