[Ninja] Ensure we compute the command hash for phony rules.
[Ninja] Ensure we compute the command hash for phony rules.

 - This was causing us to always re-check phony rules (although this didn't
   actually do anything) and update their values in the database, which had a
   serious impact on the null build time for projects with lots of phony entries
   (e.g., any large CMake project).

 - This is good for around a 50% improvement in the LLVM null build time.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild