[sanitizers] Propagate the sanitizer options through to the lit context.
This makes it *really* easy to debug leaks FYI:

ASAN_OPTIONS=detect_leaks=1 ./bin/llvm-lit -v <path to test>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207873 91177308-0d34-0410-b5e6-96231b3b80d8