llvm-cov: Accept the long forms of gcov options

This is a bit imperfect, as these options don't show up in the help as
is and single dash variants are accepted, which differs from gcov.
Unfortunately, this seems to be as good as it gets with the cl::opt
machinery, so it'll do as an incremental step.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200419 91177308-0d34-0410-b5e6-96231b3b80d8