Make every operator bool() explicit

operator bool() is a trap, but with explicit it's far safer,
and we can use that now.

Change-Id: I4e58631c94e87c00256c3ab3cff4fd2c5f632713
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/cplusplus/pp-engine.cpp

@@ -297,7 +297,7 @@ public:
         trivial.utf16charOffset = last->utf16charOffset;
     }
 
-    inline operator bool() const
+    inline explicit operator bool() const
     { return first != last; }
 
     inline bool isValid() const