[lit] Implement ulimit builtin

This patch implements ulimit inside the lit internal shell.
Implementation wise, this functions similar to umask. But instead of
setting the limits within the lit test worker process, we set
environment variables and add a wrapper around the command to be
executed. The wrapper then sets the limits. This is because we cannot
increase the limits after lowering them, so we would otherwise end up
with a lit test worker stuck with a lower limit.

There are several tests where the use of ulimit is essential to the
semantics of the test (two in clang, ~7 in compiler-rt), so we need to
implement this in order to switch on the internal shell by default
without losing test coverage.

Reviewers: cmtice, petrhosek, ilovepi

Reviewed By: cmtice, ilovepi

Pull Request: https://github.com/llvm/llvm-project/pull/157958