Make constant of enum type properly aligned

Previously, function middle::trans::trans_const result for value

```
Aaa
```

for enum like

```
enum Eee {
    Aaa,
    Bbb(u64)
}
```

was unaligned:

```
{ i8 0, [7 x i8] undef }
```

With patch applied, aligner is added to the result:

```
{ i8 0, [7 x i8] undef, [0 x i64] zeroinitializer }
```

Note, that unlike unions in C, enum members in Rust may have different
offsets, because they have different alignment. This should probably be
fixed one day (by inserting aligner before value and padding, not after),
but it requires much larger patch.

This fixes issue #13186.