CppEditor: Proliferate FilePath use

This includes one functional change: It drops some cleaning
of the path used to create the CppDocument, which is now
assumed to be done on the caller side.

Change-Id: I5e2a182028e4d5b56282ad85f4a5c665f081754f
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/libs/cplusplus/LookupContext.h

@@ -330,5 +330,4 @@ private:
 bool CPLUSPLUS_EXPORT compareFullyQualifiedName(const QList<const Name *> &path,
                                                 const QList<const Name *> &other);
 
-
-} // namespace CPlusPlus
+} // CPlusPlus