llvm-cov: Rename -color={always|never} to -use-color[=0]

This is an ugly hack to fix the configure --enable-shared build. It
turns out that *every cl::opt in LLVM* shows up in *every tool* in
that configuration, which is hopelessly broken. This skirts around the
issue by not colliding with another option's name, for now.

I've also simplified the option implementation - the other "color"
option used cl::boolOrDefault and was much nicer than what I'd written
before.

llvm-svn: 232704