auto merge of #18546 : bkoropoff/rust/unboxed-closures-cross-crate, r=nick29581

This fixes some metadata/AST encoding problems that lead to ICEs.  The way this is currently handled will need revisiting if abstract return types are added, as unboxed closure types from extern crates could show up without being inlined into the local crate.

Closes #16790 (I think this was fixed earlier by accident and just needed a test case)
Closes #18378
Closes #18543

r? @pcwalton

diff --cc src/librustc/metadata/tydecode.rs
Simple merge

diff --cc src/librustc/middle/astencode.rs
Simple merge

diff --cc src/librustc/middle/ty.rs
Simple merge