swift

3c1bcc94 - [swift_snapshot_tool] Some small fixes and add a run_toolchain command for running a command against a specific toolchain.

[swift_snapshot_tool] Some small fixes and add a run_toolchain command for running a command against a specific toolchain. run_toolchain is mainly useful for diagnosing issues with ones script without needing to fully bisect... but it also can just be used independently.