Introduced a backwards scanner for finding expression under cursor

The backwards scanner lazily tokenizes strings. This optimizes the
common case where scanning one or two lines is enough while at the same
time we extended the limit from 5 to 10 lines.

Reviewed-by: Roberto Raggi

src/libs/cplusplus/SimpleLexer.h

@@ -41,6 +41,13 @@ class SimpleLexer;
 class CPLUSPLUS_EXPORT SimpleToken
 {
 public:
+    SimpleToken(int kind, int position, int length, const QStringRef &text)
+        : _kind(kind)
+        , _position(position)
+        , _length(length)
+        , _text(text)
+    { }
+
     SimpleToken()
         : _kind(0),
           _position(0),