Fix ICE on const eval of union field

MIR's `Const::get_field()` attempts to retrieve the value for a given
field in a constant. In the case of a union constant it was falling
through to a generic `const_get_elt` based on the field index. As union
fields don't have an index this caused an ICE in `llvm_field_index`.

Fix by simply returning the current value when accessing any field in a
union. This works because all union fields start at byte offset 0.

The added test uses `const_fn` it ensure the field is extracted using
MIR's const evaluation. The crash is reproducible without it, however.

Fixes #47788

src/librustc_trans/mir/constant.rs

@@ -140,7 +140,10 @@ impl<'a, 'tcx> Const<'tcx> {
                 }
             }
             _ => {
-                const_get_elt(self.llval, layout.llvm_field_index(i))
+                match layout.fields {
+                    layout::FieldPlacement::Union(_) => self.llval,
+                    _ => const_get_elt(self.llval, layout.llvm_field_index(i)),
+                }
             }
         }
     }

src/test/run-pass/union/union-const-eval-field.rs

@@ -0,0 +1,26 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(const_fn)]
+
+union DummyUnion {
+    field1: i32,
+    field2: i32,
+}
+
+const fn read_field() -> i32 {
+    const UNION: DummyUnion = DummyUnion { field1: 5 };
+    const FIELD: i32 = unsafe { UNION.field2 };
+    FIELD
+}
+
+fn main() {
+    assert_eq!(read_field(), 5);
+}