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/libs/utils/smallstringview.h

@@ -104,6 +104,11 @@ public:
         return const_reverse_iterator(begin() - static_cast<std::size_t>(1));
     }
 
+    operator std::string() const
+    {
+        return std::string(data(), size());
+    }
+
 private:
     const char *m_pointer;
     size_type m_size;