[llvm-cov] Highlight gaps in consecutive uncovered regions

llvm-cov typically doesn't highlight gap segments, but it should if the
gap occurs after an uncovered region in order to preserve continuity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316107 91177308-0d34-0410-b5e6-96231b3b80d8