[Sanitizer] Make slow unwinder on Linux more robust

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@194805 91177308-0d34-0410-b5e6-96231b3b80d8
[asan] helper script to dump/merge coverage data

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@194809 91177308-0d34-0410-b5e6-96231b3b80d8