CPlusPlus: Use categorized logging in lexer

Not suprisingly, the #ifdef-based debugging produced uncompilable code
when enabled.

Change-Id: I4a6646bfa98a8500491be4892d464ec88512bec7
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/cplusplus/pp-engine.h

@@ -237,6 +237,7 @@ private:
     PPToken generateConcatenated(const PPToken &leftTk, const PPToken &rightTk);
 
     void startSkippingBlocks(const PPToken &tk) const;
+    bool checkConditionalNesting() const;
 
 private:
     Client *m_client;