[Coverage] Use a new API to label gap areas

This will make it possible for llvm-cov to pick better line execution
counts, and is part of the fix for llvm.org/PR34612.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@313598 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 00354796372407a06f532df2f45cc94e58f06386)
2 files changed