[lsan] Run the leak detection tests under both ASan and LSan.

Change the LSan lit test logic. Now "check-lsan" tests the leak
checking functionality in both standalone LSan and ASan.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@183000 91177308-0d34-0410-b5e6-96231b3b80d8