don't ICE when encountering an extern type field during validation

Author: RalfJung

src/diagnostics.rs

@@ -311,7 +311,9 @@ pub fn report_error<'tcx>(
             ResourceExhaustion(_) => "resource exhaustion",
             Unsupported(
                 // We list only the ones that can actually happen.
-                UnsupportedOpInfo::Unsupported(_) | UnsupportedOpInfo::UnsizedLocal,
+                UnsupportedOpInfo::Unsupported(_)
+                | UnsupportedOpInfo::UnsizedLocal
+                | UnsupportedOpInfo::ExternTypeField,
             ) => "unsupported operation",
             InvalidProgram(
                 // We list only the ones that can actually happen.

tests/fail/extern-type-field-offset.stderr

@@ -1,8 +1,8 @@
-error: unsupported operation: `extern type` does not have a known offset
+error: unsupported operation: `extern type` field does not have a known offset
   --> $DIR/extern-type-field-offset.rs:LL:CC
    |
 LL |     let _field = &x.a;
-   |                  ^^^^ `extern type` does not have a known offset
+   |                  ^^^^ `extern type` field does not have a known offset
    |
    = help: this is likely not a bug in the program; it indicates that the program performed an operation that Miri does not support
    = note: BACKTRACE: