Replace comments column with failing step names

I don't find the comments column on the LLVM status page very useful:
- For test failures, it gets truncated so it only shows the number of
  passes.
- For build failures it doesn't tell you which stage it was which
  failed.

Instead, we can show the names of the stages which are failing,
color-coded to match buildbot's conventions. I don't think this loses
any useful information compared to the comment field, and it makes it
easier to quickly see which types of failure are occuring.

Change-Id: I9a5cec73948ea6f6edc2998ef64a14704bc2c94b
1 file changed