auto merge of #15975 : dotdash/rust/unwind_lifetimes, r=pcwalton

auto merge of #15975 : dotdash/rust/unwind_lifetimes, r=pcwalton

Currently we don't emit lifetime end markers when translating the
unwinding code. I omitted that when I added the support for lifetime
intrinsics, because I initially made the mistake of just returning true
in clean_on_unwind(). That caused almost all calls to be translated as
invokes, leading to quite awful results.

To correctly emit the lifetime end markers, we must differentiate
between cleanup that requires unwinding and such cleanup that just wants
to emit code during unwinding.