swift

28ab8fb0 - [Driver] Always fall back to an arclite in the Xcode default toolchain (#22911)

[Driver] Always fall back to an arclite in the Xcode default toolchain (#22911) ...instead of the current toolchain, which we would have just searched by looking relative to the swiftc binary. rdar://problem/48044350 (cherry picked from commit e76b5a8)