[ELF] Preserve section order within an INSERT AFTER command

For
```
SECTIONS {
  text.0 : {}
  text.1 : {}
  text.2 : {}
} INSERT AFTER .data;
```

the current order is `.data text.2 text.1 text.0`. It makes more sense to
preserve the specified order and thus improve compatibility with GNU ld.

For
```
SECTIONS { text.0 : {} } INSERT AFTER .data;
SECTIONS { text.3 : {} } INSERT AFTER .data;
```

GNU ld somehow collects sections with `INSERT AFTER .data` together (IMO
inconsistent) but I think it makes more sense to execute the commands in order
and get `.data text.3 text.0` instead.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D105158