Don't include swift-latest symbolic link in available toolchains

[adam-fowler]

When you install a swift snapshot toolchain on macOS it setups up a swift-latest.xctoolchain symbolic link to point to it. We shouldn't include this symbolic link in the list of toolchains available when displaying the Select toolchain... menu

[matthewbastien]

Is there a reason you want to remove it? I feel like it's useful for people who always want to point their VSCode at the latest public Swift toolchain that they've downloaded.

[adam-fowler]

If we are going to keep it we should change the menu item to be Latest Installed Toolchain, or something similar, and verify the toolchain it is pointing to exists

[award999]

Verified with b461f8f