Fix translation of unboxing shim for rust-call ABI methods

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
Add regression test for issue #16739
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