[lsan] Add __lsan_do_leak_check() to the public interface.

Let users override the normal behavior to run leak checking earlier in
the process. Also fix a couple nits here and there.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@186581 91177308-0d34-0410-b5e6-96231b3b80d8