Modify MIR testing to require continuous lines

Mir testing now requires that lines be continuous. To achive this,
instead of collecting the expected mir as a string, it is now wrapped in
an `ExpectedLine` enum, that is either `Elision` or `Text(T)` where `T:
AsRef<str>`. `Text` lines must be matched in order, unless separated by
`Elision` lines. Matches occur greedily, that is, an Elision will skip
as few lines as possible.

To add a new elision marker. Put a comment containing only "..." and
whitespace in any MIR testing block. Like so:

```
// fn write_42(_1: *mut i32) -> bool {
//     ...
//     bb0: {
//         Validate(Acquire, [_1: *mut i32]);
//         Validate(Release, [_1: *mut i32]);
//         ...
//         return;
//     }
// }
```

Right now, all input before the line right after `// START` is elided,
and all input after the line right before `// END` is also not tested.

Many tests need to be updated. That will follow in the next commit.

cc #45153

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