llvm-cov: Rename a variable and clean up its usage

Offset is a terrible name for an indentation / nesting level, and it
confuses me every time I look at this code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217861 91177308-0d34-0410-b5e6-96231b3b80d8