CppEditor: Proliferate FilePath use

Remove SnapShot::{find,contains}(QString) overloads and fix fallout.

Change-Id: I50c415826939e09ac111f33def611c7478fa5d97
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/cplusplus/pp-engine.h

@@ -54,7 +54,7 @@ class CPLUSPLUS_EXPORT Preprocessor
     typedef Internal::Value Value;
 
 public:
-    static QString configurationFileName();
+    static const Utils::FilePath &configurationFileName();
 
 public:
     Preprocessor(Client *client, Environment *env);