Utils: Move forward declaration of SmallString in an extra header

Change-Id: I6da1cc60d425f654a31570373eb3b4f660d5f975
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/tools/clangrefactoringbackend/source/sourcerangeextractor.h

@@ -25,16 +25,12 @@
 
 #pragma once
 
+#include <utils/smallstringfwd.h>
+
 #include <vector>
 
 using uint = unsigned int;
 
-namespace Utils {
-template <uint Size>
-class BasicSmallString;
-using SmallString = BasicSmallString<31>;
-}
-
 namespace llvm {
 class StringRef;
 }