Fix `-Z print-type-sizes`'s handling of zero-sized fields.

Currently, the type `struct S { x: u32, y: u32, tag: () }` is
incorrectly described like this:
```
print-type-size type: `S`: 8 bytes, alignment: 4 bytes
print-type-size     field `.x`: 4 bytes
print-type-size     field `.tag`: 0 bytes, offset: 0 bytes, alignment: 1 bytes
print-type-size     padding: 4 bytes
print-type-size     field `.y`: 4 bytes, alignment: 4 bytes
```
Specifically:
- The `padding` line is wrong. (There is no padding.)
- The `offset` and `alignment` on the `.tag` line shouldn't be printed.

The problem is that multiple fields can end up with the same offset, and
the printing code doesn't handle this correctly.

This commit fixes it by adjusting the field sorting so that zero-sized fields
are dealt with before non-zero-sized fields. With that in place, the
printing code works correctly.

The commit also corrects the "something is very wrong" comment.

The new output looks like this:
```
print-type-size type: `S`: 8 bytes, alignment: 4 bytes
print-type-size     field `.tag`: 0 bytes
print-type-size     field `.x`: 4 bytes
print-type-size     field `.y`: 4 bytes
```

0 files changed, 0 insertions, 0 deletions