[asan] add a test which ensures that we dump coverage after SEGV

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@209295 91177308-0d34-0410-b5e6-96231b3b80d8
[sanitizer] Add __sanitizer_maybe_open_cov_file().

Summary: Add a new interface function required for coverage sandboxing support.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@209298 91177308-0d34-0410-b5e6-96231b3b80d8