Clang: Make file ids unique

Clang file ids are only unique for one query. Because we query in parallel
we have to manage our own unique ids.

Change-Id: I67d57d8b1766cab75ad252a14e57bbf9dc5fdb79
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/tools/clangrefactoringbackend/source/refactoringserver.h

@@ -30,7 +30,12 @@
 #include <refactoringserverinterface.h>
 
 #include <QTimer>
+#include <stringcache.h>
 
+#include <utils/smallstring.h>
+
+#include <future>
+#include <mutex>
 #include <vector>
 
 namespace ClangBackEnd {
@@ -65,6 +70,7 @@ private:
                                                           Utils::SmallString &&query);
 
 private:
+    StringCache<Utils::PathString, std::mutex> m_filePathCache;
     ClangQueryGatherer m_gatherer;
     QTimer m_pollTimer;
 };