Revert the test from r88984. It relies on being able to mmap 16GB of

Revert the test from r88984. It relies on being able to mmap 16GB of
address space (though it only uses a small fraction of that), and the
buildbots disallow that.

Also add a comment to the Makefile's ulimit line warning future
developers that changing it won't work.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@88994 91177308-0d34-0410-b5e6-96231b3b80d8