Pass in span to type translator, remove zombie_even_in_user_code

[khyperia]

This is pre-work to clean up the zombie system, tangentially related to fixing #274

This does two things:

• Removes the two remaining calls to zombie_even_in_user_code (other calls were cleaned up in previous PRs)
• Passes in a span to the type translator to improve error messages

The second bit causes a lot of diff spam, so I'm submitting this as a separate PR. I did so because tracking down the errors caused by the type translator are absolutely horrible to figure out without span information (it's completely opaque what's going on). Unfortunately, quite a few places don't have span information, but still, most do, and those cases are useful. Examples of places without spans are rustc_codegen_ssa APIs that ask for, say, the type for a u64, completely without context or other information. So, we avoid calling those APIs ourselves, and instead try to call wrapper methods that do take spans, hopefully minimizing the cases where that happens. DUMMY_SP is used where spans are not available.