[lsan] Add leak_check_at_exit flag.

We needed a way to tell LSan to invoke leak checking only if __do_leak_check()
is called explicitly. This can now be achieved by setting
leak_check_at_exit=false.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@187578 91177308-0d34-0410-b5e6-96231b3b80d8