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
```

src/test/compile-fail/inherit-struct8.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-test FIXME: #13991
+
+
 // Test struct inheritance.
 #![feature(struct_inherit)]
 

src/test/compile-fail/issue-9725.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-test FIXME: #13992
+
 struct A { foo: int }
 
 fn main() {

src/test/compile-fail/unsupported-cast.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-test FIXME: #13993
 // error-pattern:unsupported cast
 
 extern crate libc;