Add .swiftpm/xcode to default gitignore

[keith]

If you use the new SwiftPM + Xcode 11 integration and open the scheme
viewer, Xcode writes a shared scheme. This shared scheme ends up in a
path that wasn't previously covered by this gitignore since it's
normally intended to be checked in. We can't ignore the entire .swiftpm
directory since this can contain other files, but this ignores all Xcode
created artifacts.

This is a follow up to #2184 and fixes FB6569495

[BenchR267]

Hm, I'm not sure this is the right thing to do here (correct me if I'm wrong @aciidb0mb3r and @neonichu). The previously ignored xcuserdata/ can also be created when working with a generated Xcode project (swift package generate-xcodeproj) which we still support as far as I know.
I'm not sure about the Xcode artifacts in the .swiftpm directory but a shared scheme sounds like something you might want to share with your team colleagues and CI so I wouldn't ignore that by default.

[ikesyo]

Looks like this is a duplicate of or related to #2200.

[neonichu]

Yah, we don't want to ignore shared data by default. Unconditionally persisting schemes which have been viewed in the scheme editor is a general Xcode behaviour we are aware of, we should not try to work around it in SwiftPM.

[keith]

@neonichu so does this mean we're expecting developers to know about the project stored in .swiftpm and check in files related to it?

[neonichu]

@keith I'm assuming you mean the workspace, there shouldn't be a project.

Since that is the only way to persist edits to the autogenerated schemes, I would expect that some people need to check these in.

It is also possible that in the future, there might be more Xcode-specific configuration data that we expect developers to check in.

[keith]

I guess I'm just surprised by the UX that the workspace is transparently generated, yet developers may end up seeing untracked files related to a project that they don't have to know exists.