auto merge of #13967 : richo/rust/features/ICE-fails, r=alexcrichton

auto merge of #13967 : richo/rust/features/ICE-fails, r=alexcrichton

This change makes internal compile errors in the compile-fail tests failures.

I believe this is the correct behaviour- those tests are intended to assert that the compiler doesn't proceed, not that it explodes.

So far, it fails on 4 tests in my environment, my testcase for #13943 which is what caused me to tackle this, and 3 others:

```
failures:
    [compile-fail] compile-fail/incompatible-tuple.rs # This one is mine and not on master
    [compile-fail] compile-fail/inherit-struct8.rs
    [compile-fail] compile-fail/issue-9725.rs
    [compile-fail] compile-fail/unsupported-cast.rs
```