[llvm-cov] reset executation count to 0 after wrapped segment

Fix the bug: https://bugs.llvm.org/show_bug.cgi?id=36979. It also fixes this bug: https://bugs.llvm.org/show_bug.cgi?id=35404, which I think is caused by the same problem.

Differential Revision: https://reviews.llvm.org/D85036