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/libs/utils/fileutils.cpp

@@ -641,9 +641,9 @@ FileName::FileName()
 }
 
 /// Constructs a FileName from \a info
-FileName::FileName(const QFileInfo &info)
-    : QString(info.absoluteFilePath())
+FileName FileName::fromFileInfo(const QFileInfo &info)
 {
+    return FileName::fromString(info.absoluteFilePath());
 }
 
 /// \returns a QFileInfo