Utils: Replace FileName::FileName(QFileInfo) by a named constructor

More consistent with the fromString case and avoiding false
conversions QString -> QFileInfo -> FileName in case the
inheritance of QString suddenly disappears.

Change-Id: Ib14646ab1a660fd45dd1ea6862a0b5faa52ad0e3
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/qmakeprojectmanager/makefileparse.cpp

@@ -260,7 +260,7 @@ static FileName findQMakeBinaryFromMakefile(const QString &makefile)
                 // Is qmake still installed?
                 QFileInfo fi(qmakePath);
                 if (fi.exists())
-                    return FileName(fi);
+                    return FileName::fromFileInfo(fi);
             }
         }
     }