[asan/coverage] change the way asan coverage instrumentation is done: instead of setting the guard to 1 in the generated code, pass the pointer to guard to __sanitizer_cov and set it there. No user-visible functionality change expected

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222675 91177308-0d34-0410-b5e6-96231b3b80d8