Add a test that causes an ICE currently
rustc: Fix an ICE "Autoderef but type not derefable"

Related to #5062 (same error message), but that test case causes
a different ICE than mentioned there (even without this fix).
Add test for #5062
rustc::regionck: fix ICE when getting signature of ty_err

Fixes #5062
auto merge of #6992 : Blei/rust/fix-autoderef-ice, r=catamorphism

Related to #5062, but doesn't fix that one.