This commit fixes builtins.elemAt so it propagates catchables like
cppnix does.
Change-Id: Ieca5e128da17e78af0b14dae4a28a1ff8796e4f2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10308
Autosubmit: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
