llvm-cov: Try to appease a bot

The clang-hexagon elf bot was complaining that "Option 'color'
registered more than once!":

    http://lab.llvm.org:8011/builders/clang-hexagon-elf/builds/24425

I don't understand why this error is happening, and I don't see it on
any other bots or on my own machine, so I'm kind of grasping at
straws. Try using an unscoped enum and specifying a cl::init to see if
they help.

llvm-svn: 232698