Merge pull request #3085 from github/mbg/multi-language-repo/gitignore

Add C# artifacts to `.gitignore` for `multi-language-repo`

Author: mbg

tests/multi-language-repo/.gitignore

@@ -1,9 +1,11 @@
 .DS_Store
 /.build
 /Packages
+/obj
 /*.xcodeproj
 xcuserdata/
 DerivedData/
 .swiftpm/config/registries.json
 .swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
 .netrc
+multi-language-repo.sln