[llvm-cov] Simplify code to find the first uncovered segment. NFC.

Now that that segment builder is guaranteed to produce segments in
sorted order, we don't need a linear scan to get the right result.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313595 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed