[llvm-cov] Don't attach exec counts to lines which start a skipped region

These lines by definition don't have an execution count.

This is the final part of the fix for:
https://bugs.llvm.org/show_bug.cgi?id=34166

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312955 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed