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>
<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.
cd084abb73