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/ResolveExpression.cpp

@@ -27,6 +27,7 @@
 #include <map>
 
 using namespace CPlusPlus;
+using namespace Utils;
 
 static const bool debug = qEnvironmentVariableIsSet("QTC_LOOKUPCONTEXT_DEBUG");
 
@@ -666,7 +667,7 @@ class ExpressionDocumentHelper
 public:
     // Set up an expression document with an external Control
     ExpressionDocumentHelper(const QByteArray &utf8code, Control *control)
-        : document(Document::create(QLatin1String("<completion>")))
+        : document(Document::create(FilePath::fromPathPart(u"<completion>")))
     {
         Control *oldControl = document->swapControl(control);
         delete oldControl->diagnosticClient();