[llvm-cov] Fix a use-after-free

[llvm-cov] Fix a use-after-free

Taking a lock before appending to a vector does no good unless threads
reading from the vector also take the lock, because the vector could be
re-sized.

I don't have a good isolated test for this. I found the issue with ASan
while testing a large project.  I'm working on a bot that does this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275516 91177308-0d34-0410-b5e6-96231b3b80d8