[sanitizer] fix a performance regression in sanitizer coverage: move a small perf-critical function to a header. ALso set the coverage guard to 1 before the early return from CoverageData::Add

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@224660 91177308-0d34-0410-b5e6-96231b3b80d8