Missing "Sign in as Different User" option in sharepoint 2013

Workaround : http://siteurl/_layouts/closeConnection.aspx?loginasanotheruser=true (works on some browsers)

Permanent Fix: 

1)  C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\CONTROLTEMPLATES\welcome.ascx

2) Put this code in welcome.ascx page

<SharePoint:MenuItemTemplate runat="server" ID="ID_LoginAsDifferentUser" Text=" <%$Resources:wss,personalactions_loginasdifferentuser%>"  

Description="<%$Resources:wss,personalactions_loginasdifferentuserdescription%>" 

MenuGroupId="100"   Sequence="100"   UseShortId="true"   />