Remove outdated unreachable check from `call_visit_glue`

`call_visit_glue` is only ever called from trans_intrinsic, and the
block won't be unreachable there. Also, the comment doesn't make sense
anymore. When the code was introduced in 38fee9526a the function was
also responsible for the cleanup glue, which is no longer the case.

While we're at it, also fixed the debug message to output the right
function name.
Remove the unneeded final parameter from call_visit_glue

call_visit_glue() is only ever called with None as its last argument, so
we can remove it as well.
auto merge of #15784 : dotdash/rust/unreach, r=luqmana

`call_visit_glue` is only ever called from trans_intrinsic, and the
block won't be unreachable there. Also, the comment doesn't make sense
anymore. When the code was introduced in 38fee9526a the function was
also responsible for the cleanup glue, which is no longer the case.

While we're at it, also fixed the debug message to output the right
function name.
Implement new mod import sugar

Implements RFC #168.