Utils: Use FilePath in TemporaryDirectory API

This helps to lower impedance in the using code.

Even though TemporaryDirectory will very likely always stay on the local
host, this is one of the entry points into path related string
manipulation
that we want to base on FilePath nowadays.

Change-Id: I302016b8d65e54df94296659a54a93935d9e4627
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clanguiheaderondiskmanager.cpp

@@ -28,6 +28,7 @@
 #include <QFile>
 #include <QFileInfo>
 
+#include <utils/fileutils.h>
 #include <utils/qtcassert.h>
 
 namespace ClangCodeModel {
@@ -62,7 +63,7 @@ QString UiHeaderOnDiskManager::remove(const QString &filePath)
 
 QString UiHeaderOnDiskManager::directoryPath() const
 {
-    return m_temporaryDir.path();
+    return m_temporaryDir.path().path();
 }
 
 QString UiHeaderOnDiskManager::mapPath(const QString &filePath) const