CPlusPlus: Refactor FindUsages::getType()

Use a dedicated class instead of a collection of lambdas. We had
difficulties debugging this code, as gdb appears to have problems stepping
into lambdas. The new structure is also easier to read.

Change-Id: Icc88b5b884f1d60458c7c3254c2d13d7ab4592de
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/cplusplus/FindUsages.h

@@ -302,6 +302,7 @@ private:
     QSet<unsigned> _processed;
     TypeOfExpression typeofExpression;
     Scope *_currentScope = nullptr;
+    class GetUsageType;
 };
 
 } // namespace CPlusPlus