Memory leak when calling `span_fatal` from trans

[marijnh]

Commit 68f8812 introduces a check that can't easily be done in any pass before trans. The test case in that commit is disabled because currently it will cause a memory leak (12 objects lost) when compiled.

[brson]

This test succeeds when I modify type_needs_unwind_cleanup to always return true. There is some type here that gets cleaned up correctly via landing pads, but can't be found by the box annihilator.

[brson]

type_needs_unwind_cleanup is completely broken I believe. Oops.

[catamorphism]

I think I just got bit by the same bug -- see the above-referenced commit. In my case I was calling span_fatal from typeck. The fix @brson suggested fixed it as well, but rather than checking in that fix, I just xfailed the test (class-implements-int), since @brson said he was going to work on it tomorrow.