auto merge of #18113 : bkoropoff/rust/issue-16739, r=alexcrichton

When translating the unboxing shim, account for the fact that the shim translation has already performed the necessary unboxing of input types and values when forwarding to the shimmed function.  This prevents ICEing or generating incorrect code.

Closes #16739

diff --cc src/librustc/middle/trans/type_of.rs
Simple merge