Add a script to setup codesigning on macOS.
Add a script to setup codesigning on macOS.

I've been using this script on a couple machines and it seems to work
so I'm putting it out there, maybe other people will find it useful.
It is strongly inspired from a similar script in the delve project.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@334743 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb