Were are the ascx files used in custom visual webparts in the 15 hive?
They used to be in the
If you are talking about creating custom visual webparts, the ascx file will not be in the 15 hive as it was in SP2010, the ascx is generated by the webpart's dll when the code is executed, it's not going to be available in the 15 hive.
Therefore, the ascx file itself is not necessary and will not be deployed.That is the reason we never find System.Web.UI.UserControl added on MyWebPart.ascx.g.cs file which is autogenerated by Visual Studio. If you want short development cycles, you can use tools like CKSDev that allow a quick deployment of the compiled assembly to the GAC (without deploying the whole solution). This is not as fast as editing and copying the ascx-file, but it is faster than waiting for the whole solution to be deployed by SharePoint.
They are in C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\CONTROLTEMPLATES