In fact, you can just use regular Mac-Shortcuts.
Start line
cmd + shift + ->
Midline
1. cmd + -> (this will lead you to the end of the line)
2. cmd + shift + <- (selects everything to the left)
End of line
cmd + shift + <-
Hope this helps :)
Update
In Xcode 8, I found a Select Line keyboard binding that you can set to cmd-l , for example, so that Xcode selects the line you are currently on when you click this shortcut.

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