Update an obsolete comment about conditions
auto merge of #13399 : SimonSapin/rust/patch-8, r=cmr
Make constant of enum type properly aligned

This patch fixes issue #13186.

Previously, function `middle::trans::trans_const` result for expression:

```
Aaa
```

from enum:

```
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 }
```

Currently aligner is an empty array of appropriate integer size.
Mapping from alignment to arbitrary aligner is computed at compilation
start and stored in struct CrateContext.

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