MIR dump const Allocation printing of large relocation ids

[oli-obk]

After #69916 the MIR dump of const Allocations can result in things like

    0x80 │ 00 00 00 00 00 00 00 00 00 00 ╾alloc11+0╼ 00 00
    0x90 │ ╾alloc12+99 (4 ptr bytes)╼ 00 00 00 00 00 00 00 00 00 00 00 00

where even just the alloc12+99 would be too wide on 32 bit systems.

We should find better ways to handle this. Some suggestions:

1. Indent all columns so they align again after the oversized relocation

    0x80 │ 00 00 00 00                00 00 00 00 00 00 ╾alloc11+0╼ 00 00
    0x90 │ ╾alloc12+99 (4 ptr bytes)╼ 00 00 00 00 00 00 00 00 00 00 00 00

2. More ascii art

    0x80 │ 00 00 00 00 00 00 00 00 00 00 ╾alloc11+0╼ 00 00
    0x90 │ ╾─────┬───╼ 00 00 00 00 00 00 00 00 00 00 00 00
          ┌──────┴─────┐
          │ alloc12+99 │
          └────────────┘

[jclarkson]

Hi, I would like to have a go at this.

[jclarkson]

@rustbot claim

[dtolnay]

@rustbot release-assignment