Utils: Make SmallString(std::string) a template

Normal contructors are called before templated constructors. So it would
call SmallString(std::string("foo)) for SmallString("foo"). To prevent that
we change it to a template constructor.

Change-Id: Ibd249cedd638cbc2e6755c988be41fa5bb6e1ff6
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/libs/utils/smallstring.h

@@ -128,8 +128,11 @@ public:
         : SmallString(SmallString::fromQString(qString))
     {}
 
-    SmallString(const std::string &stdString)
-        : SmallString(stdString.data(), stdString.size())
+    template<typename Type,
+             typename = typename std::enable_if<
+                 std::is_same<typename std::decay<Type>::type, std::string>::value>::type>
+    SmallString(Type &&string)
+        : SmallString(string.data(), string.size())
     {}
 
     template<typename BeginIterator,