llvm-cov: Make debug output more consistent

This changes the debug output of the llvm-cov tool to consistently
write to stderr, and moves the highlighting output closer to where
it's relevant.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217838 91177308-0d34-0410-b5e6-96231b3b80d8