[lsan] Add lit test support.

"check-lsan" now runs both the tests from lib/lsan/tests and any lit
tests found under lib/lsan/lit_tests.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@182583 91177308-0d34-0410-b5e6-96231b3b80d8