tree bc31fbe65f2a1434abfc5706ae136b9676bb1693
parent 3e4ba132c7ab93da051f27a49064b5cca9cad6a0
author Daniel Dunbar <daniel_dunbar@apple.com> 1484778738 -0800
committer Daniel Dunbar <daniel_dunbar@apple.com> 1484780463 -0800

[BuildSystem] Add support for "command timestamp" nodes.

 - These nodes can be used to build relationships between commands which
   effectively cause one command to trigger another to build any time it
   updates.

 - This can be used as part of a primitive mechanism for supporting commands
   which need to mutate the outputs of other commands, by treating the output as
   effectively one of the mutating command, and using a trigger relationship
   between the producing & mutation commands to cause them to run in concert.