Rollup merge of #131121 - lqd:dataflow-viz, r=compiler-errors

A couple of fixes for dataflow graphviz dumps

A couple of trivial drive-by fixes to issues I noticed while debugging my buggy borrowck code:

One is a fix of the `-Zdump-mir-dataflow` file extensions, the dataflow graphviz files are currently dumped  as `..dot`.

<details>

```console
-rw-rw-r-- 1 lqd lqd 13051 Oct  1 23:21 mir_dump/issue_47680.main.-------.borrows.borrowck..dot
-rw-rw-r-- 1 lqd lqd 13383 Oct  1 23:21 mir_dump/issue_47680.main.-------.ever_init.borrowck..dot
-rw-rw-r-- 1 lqd lqd 13591 Oct  1 23:21 mir_dump/issue_47680.main.-------.maybe_init.borrowck..dot
-rw-rw-r-- 1 lqd lqd  9257 Oct  1 23:21 mir_dump/issue_47680.main.-------.maybe_init.elaborate_drops..dot
-rw-rw-r-- 1 lqd lqd 14086 Oct  1 23:21 mir_dump/issue_47680.main.-------.maybe_uninit.borrowck..dot
-rw-rw-r-- 1 lqd lqd  9257 Oct  1 23:21 mir_dump/issue_47680.main.-------.maybe_uninit.elaborate_drops..dot
```

<summary>Some examples on nightly</summary>

</details>

And the other is for the specific `Borrows` dataflow analysis, whose domain is loans but shows locations when dumped (the location where the loan is introduced). It's not a huge deal but we didn't even print these locations in MIR dumps, and in general cross-referencing loan data (like loan liveness) is more annoying without this change.

<details>

![Untitled](https://github.com/user-attachments/assets/b325a6e9-1aee-4655-8441-d3b1b55ded3c)

<summary>Here's how it'll look in case inquisitive minds want to know</summary>

</details>

The visualization state diff display is still suboptimal in loops for some of the effects escaping a block, e.g. a gen that's not dominated/postdominated by a kill will not show up in statement diffs. (This happens in the previous screenshot, there's no `+bw1` anywhere). We can fix that in the future.

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