Avoid storing references to rvalue FileName strings

Following 73616849, it is dangerous to store a reference to the rvalue of
toString(), since it might become dangling.

Example:

FileName foo() { return FileName::fromString(QLatin1String("/some/file")); }

void func()
{
    const QString &fileName = foo().toString();
    // fileName is now a dangling reference
}

Change-Id: I5dfad5dc8dd568a0a3c8f9f71ad93292dc26cbbe
Reviewed-by: hjk <hjk@theqtcompany.com>

src/plugins/projectexplorer/session.cpp

@@ -490,7 +490,7 @@ QStringList SessionManagerPrivate::dependenciesOrder() const
 
     // copy the map to a temporary list
     foreach (Project *pro, m_projects) {
-        const QString &proName = pro->projectFilePath().toString();
+        const QString proName = pro->projectFilePath().toString();
         unordered << QPair<QString, QStringList>(proName, m_depMap.value(proName));
     }